'I was just amazed. I couldn't work out whether to get out of the car and run and help, or move my vehicle out of the way of the emergency services. I decided to do a three-point turn and clear out of the way,' Somerville added.

Even for a trained doctor, the scene confronting Somerville was almost too much to comprehend. Blood was splattered up the walls of the BMA's offices. A blue plaque nearby declaring that the building had once been home to Charles Dickens was pockmarked with shrapnel. The bus looked grotesque. Along one side an advert for a forthcoming film could still be made out. 'Outright terror, bold and brilliant,' the ad ran. The rest of the ad had been blown away.

The carnage contrasted brutally with the genteel tranquility of the nearby square with its dedications to peace - a statue of Mahatma Gandhi, a Hiroshima cherry tree and a Holocaust memorial.

Dr Sam Everington was inside the BMA's headquarters when he heard the blast. Everington immediately knew what it was and ran downstairs to see bodies scattered across the pavement. With six other doctors he immediately began trying to save the lives of the casualties, but it wasn't easy because they only had the most basic equipment.

Everington, a GP who has spent his life trying to improve healthcare for the most deprived communities of east London, had never dealt with such an emergency, but he knew what had to be done: keep the seriously injured alive until the paramedics arrived.

'The majority of casualties were all over the road but there was one person alive on the bus. He was Chinese, and we were worried about a neck injury so a passer by helped us by holding his head in the right position for quite a long time, until the fireman got him out. Next time I saw him he was in BMA House, seriously injured. I don't know what happened to him.'

The doctors had to try and resuscitate the victims with whatever equipment they could. Dishcloths, tea towels and tablecloths were used to try and stem the enormous blood losses.

After 20 minutes of treating people at the roadside, the doctors were warned by police there might be another bomb on the bus. They had to evacuate the area, and brought boards and tables out of BMA House, using them to carry people into their courtyard, where a makeshift casualty unit was established.

'Much of the work was simply about stopping bleeding. A bit later, we had access to fluids and oxygen, but it took quite a long time for the emergency services to arrive because this was the last bomb blast and most of them were at the other sites and couldn't get through easily,' Everington said. Some had lost limbs; a turniquet had to be used to save one person who lost the lower part of a leg. Another person died as the doctors worked.

'It was simply about trying to stop the bleeding, keeping the airway open, and dealing with chest injuries.'

Amid the carnage workers on a building site nearby rushed to give blood. Cordons were erected around the square and the roads leading to the nearby hospitals of Great Ormond Street and University College. Staff acted as human barriers to prevent traffic blocking up the streets so the ambulances could get through.

Up to 100 members of the Church of Scientology in garish yellow jackets joined the relief operation as sandwiches and bottles of water were rushed through the police cordons. As the mobile phone networks collapsed under the weight of calls people queued outside phone boxes to call relatives.

As the ambulances ferried the dead and the injured away teams of forensic experts descended on the square. The painstaking task of collecting evidence was repeated at the other three blast scenes as the forensic teams looked for clues. Underneath Russell Square anti-terrorist officers battled against horrific conditions as they tried to gather evidence while emergency services attempted to recover bodies trapped in the wreckage. Their work was hampered by stifling heat and the threat of a tunnel collapse.

With so much of the horror contained underground, the scenes at Tavistock Square became the focus of the world's television cameras. The twisted wreckage of the bus became the gruesome emblem of London's darkest day since the Second World War.

Among all the memorable images shown on television in the evening evening following the bus blast, one stands out for depicting the urgency - and the terror - of the passers-by in Tavistock Square. It is of a young couple holding hands, running away from the explosion.

This weekend Joe Bor and Vicki Dokas cannot believe how close they came to death. They had been standing 20 yards from the bus as it slowed down on its unfamiliar route through Russell Square. Bor, a storyboard artist for a film production company, was on his mobile explaining to his work he would be late in. Vicki was standing next to him.

'Nothing was really moving, and there was a lot of traffic, a lot of people like us who had started walking from Euston or King's Cross to try and get to work,' Bor recalled. 'As I put the mobile phone down, I heard a massive bang and I felt this enormous power hit me in the face. All I could see was white smoke and then the top of a bus, which was opened up like a tin can.

'I didn't know what to do. I froze, but my girlfriend shouted out, "Run".' Bor obeyed and then he looked back in order to grab her hand. He saw some thing he will never forget. 'As the smoke died down, there were bodies everywhere. Everyone I could see in front of me looked dead. Suddenly, after all the noise of people trying to get to work, it was very, very silent. There was blood splattered everywhere, but strangely people seemed quite calm.'

He found himself caught in a disturbing dilemma. 'I wanted to take a photo, to document it, but then I didn't want to, because it seemed wrong. Then people started crying, my girlfriend was hugging a woman who was very distraught. Many people started to shelter in doorways, I don't really know why. We tried to, but the doorways were full up, so we just walked away.'

'Catastrophic emergency'

Chief Superintendent Chris Allison had just had a busy six days. As 'Gold' commander of the Metropolitan Police he was in charge of policing the Live8 concert at Hyde Park the previous Saturday and ensuring that any anti-globalisation protesters passing through the capital on their way to Gleneagles did not cause any trouble.

Arriving at his office early on Thursday morning Allison could reflect on a job well done. But he had little time to do so. Shortly before 9am, Allison was told of a number of incidents on the underground. Although there had been no confirmation of any bombs at that time, he moved straight to the second floor of New Scotland Yard's headquarters and set up the so-called GT operation room. 'My initial thoughts were, "Could it be a terrorist attack?" Since 9/11 we had to be prepared for something like this,' Allison said.

Minutes dragged by as Allison tried to get a clearer picture of what was happening. But it wasn't until a fellow officer told him, 'We've got a bus that's been blown up,' that he knew for certain London was facing the nightmare scenario: a co-ordinated and well-planned terrorist strike at the height of rush hour.

Allison's team kicked into action, implementing the emergency contingency plan they had meticulously prepared for such an horrific occasion.

The Cabinet Office Briefing Room A (Cobra) - Britain's civil contingencies committee based in the bowels of Downing Street - was activated and a video link with Allison established. In the absence of the Prime Minister, who normally chairs the committee but who was in Gleneagles for the G8 summit, the committee was chaired by the Home Secretary, Charles Clarke.

Whitehall immediately classified the events unfolding across central London as a Level Three incident - a 'catastrophic emergency'. Initial discussions covered whether to summon the 7,000 strong Civil Contingencies Reaction Force, made up of reservists, to the streets of London. Despite the chaos in central London, it was not deemed essential and control fell to the police.

As the drama unfolded Allison watched the bank of screens beaming images from around the capital and saw passengers emerging from the tube bleeding and in tears. The only positive news came from the London Fire Brigade who were the first on the scene and were not reporting any evidence of a chemical or biological attack. 'Very early on we had indications we were dealing with conventional explosives,' Allison said.

Nevertheless many of the 1,500 gas-tight suits available in London for use in a biological and chemical had to be deployed just in case. Atmospheric samples had to be analysed before the rescue operation could begin, a process that helped explain why commuters were left stranded for minutes in the dark following the attack.

On the ground floor of the Department of Health, the emergency incident room had been activated and was receiving minute-by-minute updates from the ambulance service: operations were cancelled and beds cleared in the 13 hospitals designated to take casualties, with more standing by in case of overflow.

Blood stocks were readied for what could be multiple transfusions to the injured, and a major conference of plastic surgeons in Reading was abandoned as doctors sped back to the capital to help with burns victims.

Inside Cobra, Cabinet ministers were still trying to calculate 'the scope and scale of what had happened,' according to Clarke. With anxious commuters now stranded across London, unable to communicate by mobile phone as the emergency services commandeered airtime, the next challenge was what to tell the public. There was a brief clash in the meeting over whether to call an immediate press conference: Prescott argued successfully that they simply did not know the answers to the inevitable questions. It was not even clear whether the attack was over yet.

Minutes later, as Cobra broke up, Clarke first made a brief conference call to Blair, who was by then making plans to fly back to London, before delivering a brief statement on the steps of Downing Street. Condemning a 'criminal and appalling' act, he refused to give any casualty figures or speculate on the extent of the attack. In truth, with the police now overwhelmed by calls from nervy Londoners reporting anything that looked suspicious, and the most reliable intelligence coming from ambulance callouts to what at least were confirmed casualties, he did not yet know for sure.

Meanwhile Allison held a meeting with the heads of all the emergency services, local authorities and transport bodies. 'Our first priority was to preserve life and deal with the casualties. Secondly we had to preserve evidence and arrest any suspects. Finally we had to provide reassurance to the public and the business community.'

Allison's job was to keep the Cobra committee informed of what was happening. Fortunately all the preparation for a terrorist incident seemed to be working. 'After the initial shock London pulled together,' Allison said. It was to prove a remarkably efficient operation, all the more impressive considering the emergency and security services had no prior warning of the attacks.

Even the intelligence services seemed to believe the chances of an attack were not high last Thursday. Less than a month ago the Joint Intelligence Committee had been briefing the Cabinet that there had been a reduced level in the perceived threat to the UK from terrorist cells.

Although the general level of threat remained unaltered, the UK's intelligence agencies, and those working on behalf of friendly governments, had compiled a picture that suggested there had been a lowering of the risk. So-called 'chatter' - the conversations between terrorist and extremist groups about potential operations, monitored by both the government's secret listening centre at Cheltenham, GCHQ, and the US National Security Agency, had significantly diminished in the preceding months.

The focus on 'chatter' was a relatively recent phenomenon for the intelligence services. Even as late as 1998, when an al-Qaeda team set off two massive bombs and demolished two American embassies in east Africa, analysts still felt that Britain was not a target.

But investigations into that attack had found key figures connected to al-Qaeda had been based in Britain for years. A series of operations were launched. In 2001, an alleged logistics specialist for the terrorist group was arrested in the UK.

The attacks of 11 September changed everything. In the aftermath of the strike in America, the security services poured everything they had into tracking the new threat. The Prime Minister told MI5 he did not want lengthy surveillance operations but swift action against any potential threat.

At first it was felt the threat came from overseas, in the shape of committed al-Qaeda cadres being sent into the country for specific operations. Then, once the fragmented, decentralised nature of the organisation became clear, the worry was that new immigrants or asylum seekers might be the main menace. Scores of arrests and detentions under new legislation - though few convictions - followed. Then, the focus changed again. The stories of the 'Tipton Taliban' and Britons like Richard Reid, the shoe bomber, indicated to specialists that British citizens might be more dangerous than any foreigners.

Surveillance of mosques and prisons, two of the most common locations for radical recruitment, was stepped up. As the controversial Iraq war dragged on, analysts distinguished three main sources of threat: 'classic' al-Qaeda operatives, home-grown militants with no experience and Britons, or other Europeans, who had travelled to Iraq and were returning to their homelands. The latter threat, so called 'Bleedback' from Iraq, was regarded as the greatest threat.

Grim news around cabinet table

As they set off through the drizzle to Downing Street for their routine weekly meeting last Thursday, Tony Blair's Cabinet had little idea of what the day would bring. News of an apparent crash on the London Underground a few minutes before had already started to filter through, but it was not until they were gathered around the Cabinet Room's coffin-shaped table, and the urgent messages began pouring in to John Prescott - chairing the meeting while Blair was in Scotland for the G8 summit - that the awful truth began to sink in. The moment they had dreaded ever since 11 September had arrived: London was under attack.

It could not have happened at a more vulnerable time. Blair was in Gleneagles, buried in talks with the Chinese as several key security personnel and hundreds of Met police officers were in Scotland protecting the summit.

The first hint something terrible was underway came to Blair's inner circle - including his director of communications David Hill and chief of staff Jonathan Powell - just before 9.30am. Watching TV in a hotel suite in Gleneagles, hairs rose on the backs of necks.

'It started to seem slightly too random: something wasn't right,' said one senior Downing Street source. Almost immediately, the first message came in from Downing Street confirming there had been explosions.

A scribbled note was immediately dispatched to Blair, who came out of the meeting to be briefed - although there was precious little information to give him. The first decision was whether to make an immediate broadcast to the nation, but with little detail to go on, there were fears that would simply cause panic. In the room itself, there was grief but an odd kind of calm.

'He wasn't shocked: he was distressed, but you could immediately see the resolve,' says one source close to him. 'It's always in the back of his mind, in all of our minds, that something might happen. He knew what he had to say.'

Down at Westminster wild rumours - none of them true - were doing the rounds: a police sniper had shot a would-be suicide bomber at Canary Wharf; troops were to be put on the streets; the casualties were 'higher than Madrid', when 191 died.

In the cavernous Gothic lobby of the House of Commons, attendants crowded round a television screen hastily tuned into live coverage of Gleneagles. Head bowed, visibly struggling with emotion, Tony Blair denounced as 'barbaric' the timing of an attack coinciding with the talks on global poverty, vowing that the British people would not be defeated by terrorism.

Prayers from around the globe

For one visitor to London last Thursday the day's events were gruesomely familiar. Former Mayor of New York, Rudolph Giuliani, was only yards away from the first explosion and memories of 9/11 quickly came to mind.

'They are a very eerie reminder of 11 September. I was right near Liverpool Street station when the first bombing took place, so I could hear the sirens and then kept hearing reports of different bombing, in different parts of the city.

'As we were walking through and driving through the streets of the city, it was remarkable how the people of London responded calmly and bravely,' Giuliani said.

As news spread of the atrocities, religious leaders lined up to condemn the bombings. 'The appalling events in London this morning have shocked us all,' said the Archbishop of Canterbury, Dr Rowan Williams.

The Pope said he was 'deeply saddened' by the events. In a message sent to Cardinal Cormac Murphy-O'Connor, head of the Catholic church in England and Wales, Pope Benedict XVI offered 'fervent prayers' for victims of the blasts.

The Muslim Council of Britain issued a statement saying it 'utterly condemns the perpetrators of what appears to be a series of co-ordinated attacks'.

Police forces across the UK were put on a heightened state of alert. Cities across the globe expressed sympathy and solidarity with London while simultaneously increasing their own security provisions.

As the day wore on a clearer picture of the scale of the evacuation operation was to emerge. Gareth Davis, the medical director of London's air ambulance service, said the emergency services had managed to get more than 10,000 people out of the underground system.

'There were walking wounded, people were scattered everywhere when we arrived. The more serious injuries were treated on the track; we had all the equipment necessary to perform medical procedures.

'There were seven teams of doctors divided over the four scenes; it was difficult to move around due the number and size of explosions,' Davis said calmly. The comments seemed to capture the tone of the day: on every street there was quiet stoicism in the face of horror.

Across London people had quietly got on with their lives. A weird calm prevailed as the morning's showers were replaced by the hot sun of the afternoon. Paparazzi on rollerblades swooshed by. There was not a traffic warden to be seen nor a pneumatic drill to be heard.

Uncertain of how they were going to get home, people started to gather around TVs in pubs to watch the news. Unable to return to their hotels located around Tavistock Square, scores of American tourists stood watching the drama from afar as small pockets of rubber-neckers turned up to gawp.

A few hundred yards away thousands of people walked across central London's usually busiest roads which by now were denuded of traffic, the result of the police closing off the capital's main arteries.

At the back of University College London Hospital there was an ominous portent. A lorry laden with oxygen bottles trundled into the bowels of the hospital's underground carpark, fulfiling an urgent demand for extra supplies.

Forensic search for minute clues

Friday morning brought forth a terrifying question. Had Britain experienced its first suicide bombing? An eyewitness on the number 30 bus said he had seen a man in his mid twenties, with 'olive skin', acting strangely. 'This chap started dipping down into his bag and getting back up,' said Richard Jones, an IT consultant. 'He did it about a dozen times in two or three minutes and looked extremely agitated.'

A nurse at UCLH said some of the injured on the bus had talked of how the man had descended the stairs and then exploded.

Then came another question. Could the bombings have been the work of one person? All the tube trains had to travel through the Kings Cross area.

Conceivably a terrorist could have placed devices with timers on the trains before boarding the number 30 bus when he or she either intentionally or accidentally detonated the explosive.

Just metres away underground, engineers were battling to release the bodies of the dead on Piccadilly tube train. The carriage where the bomb had gone off was some 500 yards into the tunnel and the engineers were finding it difficult to gain access. 'It was like a scene from hell,' said one.

But the job could not be rushed. The forensic experts knew that every shard of glass, every splinter of metal or scrap of clothing was potentially crucial. Fibres no more than five to 10mm in length and a few microns - millionths of a metre - wide could hold the key to the perpetrators' identities.

Officials from the government's Forensic Science Services (FSS), whose headquarters are less than two miles from Tavistock Square, removed thousands of samples for analysis at its state-of-the-art laboratory in Birmingham.

The forensic investigation may take weeks to complete. Among the most sought-after items are the actual timers and detonators of the devices. Although terrorist bombs are constructed in different ways, the timers and detonators usually survive the explosion with the result that, normally, three-quarters of a bomb can be recovered from the scene.

Already initial images of the four bombings have been fed to experts at Qinetiq, the government's former research agency, which will begin creating computer simulations to show the precise location of where the bombs were planted and direction of the blast.

This week, focus will move towards locating the CCTV camera in the number 30 bus. Yesterday the Metropolitan police were refusing to confirm whether it had remained intact or even if it was recording images on Thursday morning. Sources later said that it seemed that the CCTV camera was not working. This week it is likely the bus will be transferred to a 'secure' Metropolitan Police site for detailed forensics work.

Investigators believe it will be vital clues from the bus that will give them the best clues in the hunt for the killers. They recall that it was a finger print on the SIM card of a mobile phone that led to the Madrid bombers.

Evidence from passengers will also be crucial. Trained police officers were very quick to ensure passengers leaving the tube trains gave statements about what they saw.

Yesterday a more detailed picture started to emerge of what had happened. It is now accepted that the type of explosives used were not amateur or hand-made, but small 10lb commercial high explosives linked to a timing device.

The fact that it has now been established that the tube bombs detonated almost simultaneously and were laid on the floor close to the doors almost certainly rules out the prospect that the underground attacks were by suicide bombers or someone acting alone. MI5 are believed to be focusing on almost 100 suspects.

But if, as now seems likely, the terrorists were acting as a team, the key questions are where did they come from and where are they now?

The theory that the bombers were young, radicalised, British-born Muslims is the one causing most concern among police. One line of inquiry concerns 'holy warriors' travelling to Europe from the insurgency in Iraq. Sources confirmed the security services had traced people back to Britain who had fought in Iraq.

The security services said: 'We are looking at under 100 people since the start of the war in Iraq war going from Britain. Our concern is when the pipeline starts to flow the other way.'

Two recent anti-terrorist operations overseas involved breaking up alleged terror cells comprising British Muslims. One al-Qaeda operative arrested in Pakistan had maps of the London underground on his computer.

Intelligence officials also believe that more established jihadist groupings have developed in the way they conduct their operations, spending less time on the ground and in more temporary groups brought together for specific operations.

'What is almost impossible to cater for,' said one source, 'are groups that come together just for one attack, especially if they are observing tight security in their communications.'

What investigators are certain about, however, is that an operation like Thursday morning's requires external support of some kind.

The group that first claimed responsibility for the attacks, the Secret Organisation of the al-Qaeda Jihad in Europe, claims to be linked to Abu Musab al-Zarqawi, the notorious terror chief in Iraq who is leading the bloody insurgency operations who is believed to be behind the kidnap and murder of Britons Ken Bigley and Margaret Hassan in Iraq.

This weekend intelligence experts from Spain arrived to help with the London investigation focusing on similarities between the two attacks. The Spanish security services found concrete links between Islamic fundamentalist groups in Madrid and militants who lived in north London. Several of the suspects from the Madrid cell are still believed to be at large.

Hunting down the murderers

Tomorrow, Tony Blair will make his first detailed statement to the Commons on the hunt for the bombers - and the mission to help the bereaved. Blair's natural feel for the pulse of a nation in a crisis, and his avoidance of the mistakes Bush made after 9/11 - such as being whisked away to a safe bunker while others were exposed to attack - have shielded him from instant recrimination.

But the nagging fear for Downing Street remains: what if Londoners decide that Blair's war, the assault on Iraq, has made them a sitting target for Islamic extremists? Without the war, would the attacks on Thursday have happened?

So far, only one politician has levelled such allegations. Fresh from four hours of watching the bloodied survivors being tended at his local Royal London hospital, the anti-war maverick MP, George Galloway, argued the bombing of London had been 'entirely predictable' and that the 'bitterness and emnity' generated by the Iraq and Afghanistan wars fuelled support for Osama bin Laden.

Downing Street sources yesterday dismissed the outburst as provoking feelings 'little short of revulsion', warning it could backfire with devout Muslims now among the missing and feared dead.

Galloway's argument was, however, based on a warning from the Joint Intelligence Committee - revealed only after the war - that the threat from al-Qaeda 'would be heightened by military action against Iraq'. Made by a more mainstream politician, this claim could yet cause significant damage.

The second threat to Blair is an accusation of negligence. Downing Street insists the initial priority for the intelligence services is to find the bombers, not to investigate their own failure to predict the attack: Blair is understood not to blame the intelligence services.

But a review is inevitable, focusing on the downgrading of the al-Qaeda threat. Had the intelligence services become complacent - or had the Government taken its eye off the ball?

Certainly, many politicians had started to believe the threat was less becoming acute: last month three Conservative backbenchers accused Blair of 'excessive' personal security as he drove the few yards from Downing Street to the Commons.

Peers, too, have grumbled loudly about the ugliness of the concrete blocks outside the Lords designed to deter suicide bombers.

Such complaints will be silenced now as the messages from the missing still flutter unanswered in the winds around King's Cross, and London sets on the long journey of healing wounds.