Curling in the wind outside King's Cross underground station yesterday, posters pleading for information on the whereabouts of the missing told stories of loss and devastation.

The scene was reminiscent of the Wall of Tears which appeared in Manhattan after 11 September 2001 and in Thailand after the tsunami of Boxing Day 2004. For most Londoners these were disasters in faraway places understood via television. Now death was on the doorstep.

Until now, London's name had been missing from the chronicle of this new era of global terrorism. In New York, Washington and Pennsylvania, 2,948 people were killed. In Bali, in Indonesia, 202 died. In Istanbul, the death toll was 61. In Madrid, 191 people lost their lives. In London, more than 50 are confirmed dead, with the figure likely to rise by another 20. It is a measure of the changed temperature that many in the British capital last week found their shock at such an aggregate balanced by relief: it could have been, and was always thought likely to be, significantly worse.

When 29 people were killed in the Omagh bombing it appeared that terrorism had reached a new, despicable, low. Now the Twin Towers, Bali and Madrid act as some type of anaesthetic: 70 is seen almost as a blessing. Ridiculous, of course. Meaningless for the mothers and fathers, husbands and wives and everyone else directly touched by last week's events. And yet the story of Thursday's bombings is not simply one of unremitting grief and destruction. Out of the carnage remarkable tales of heroism and compassion have emerged. Politicians, religious leaders and the public have united to condemn the atrocities. Far from spreading terror, the bombings seem to have had the opposite effect, a way of uniting the capital in the face of incomprehensible adversity. This is the story of London, Thursday 7 July 2005 and its aftermath.

It started innocuously enough. The reports came through just before nine o'clock. There was some kind of small scale explosion on the underground, probably due to a power surge. The details were sketchy. Maybe a few people injured; British Transport Police were investigating. Above ground London went about its normal business, the capital basking in the glory of having won the bid to host the 2012 Olympic Games the previous day.

But underground a horrific narrative was starting to unfold. Terrified commuters were choking on dust in the dark. An explosion had ripped through the floor of the third carriage of a tube train as it was coming into Liverpool Street station. It was 8:51 AM. The tube, travelling on the Circle line from Aldgate station, was packed with commuters.

George O'Connell, 16, was on his first day of work experience in the City. 'I remember the moment the blast happened, I saw white light, I thought I was dead. I heard a massive bang, I was covered in blood. The doors blew off from the force of the blast. People were screaming get us out.'

'We felt the train shudder,' said another passenger, Terry O'Shea. 'Then smoke started coming in to the compartment. It was terrible. People were panicking, but they calmed down after one or two minutes.' Laser pens were used as torches. Fire extinguishers were used to break down doors. Lights on the tube flickered on and off. People sobbed. Some screamed. Others were silent, struck dumb by the terrifying noise of the explosion.

As the dust settled reassuring voices came over the speaker system urging the passengers to stay calm as an evacuation process was introduced. 'As they led us down the track past the carriage where the explosion was, we could see the roof was torn off it, and there were bodies on the track,' O'Shea said.

Loyita Worley, 49, was in a neighbouring carriage when the explosion struck. 'There were people with blood dripping off them. Eventually they opened up the front of the carriage. We walked along the track in between Aldgate and Liverpool Street.'

The emergency services were on the scene within three minutes, quickly prioritising the walking wounded, slapping labels on them with quiet efficiency, something they had practised during numerous dress rehearsals, only this time it was now being played out in grotesque reality.

As stunned commuters, many caked in soot and dust emerged above ground, horror stories started to filter out. Steve Nichols, a London Underground chaplain, found himself counselling traumatised members of the emergency services in St Botolph's Church near Aldgate East station. Their stories are likely to haunt him for a long time. 'There was one poor lady who had been impaled by one of the poles in the train and she was still alive. The people said they had been picking up various body parts,' Nichols said.

Heba Al-nasria, 24, a junior doctor from Iraq working at the Royal London hospital in the east of the capital, found herself in a strange situation. 'I received a call from my family in Iraq; they were concerned for my safety. They said they were used to living with constant fear; it is something London could more than cope with.'

Normally Al-nasria would find herself working in a ward with 12 cubicles with one patient per cubicle. But on Thursday more than 45 people were being treated in the same area. 'There were the walking wounded, people with burst eardrums. It was supposed to be my day off, but my conscience would not let me stay at home when people required my help.'

Meanwhile, just over a mile away, another subterranean nightmare was unfolding. A second device, in the first carriage of a tube travelling on the Piccadilly line between Russell Square and King's Cross, had detonated only moments after the explosion in the east of the capital. The depth of the explosion, 30 metres underground, was to make the subsequent rescue operation all the more difficult for emergency services.

'It was about three minutes after we left King's Cross when there was a massive bang and there was smoke and glass everywhere,' said one of the commuters on the tube, Fiona Trueman, 26. 'The lights went out, and with the smoke, we couldn't breathe. We sort of cushioned each other during the impact because the compartment was so full. It felt like a dream, it was surreal. The screaming from the front carriage was terrible. It was just horrendous, it was like a disaster movie, you can't imagine being somewhere like that. You just want to get out. I kept closing my eyes and thinking of outside,' Trueman said.

In the darkness, Spanish hotel worker Gemma Signes, 32, waited for news. The communication system seemed to have failed. 'It was pitch black, you couldn't see anything,' Signes said. 'Everyone was screaming and panicking. No one knew what to do. There was smoke everywhere - I could hardly breathe. But someone managed to get the doors open because people started pushing out into the tunnel.'

At last the driver's voice crackled over the communication system. 'He talked to everyone and was wonderful about it. After a few minutes we got out and walked along the tracks. I saw a few people who were in a bad way,' said another passenger, Ellie Thompson.

Emergency crews started to arrive and unloaded the passengers from the back of the train from where they walked the 15-minute journey along the track back to King's Cross.

Above ground the Salvation Army centre opposite King's Cross was rapidly turned into a makeshift aid centre as emergency services swarmed around the scene. Survivors were quick to share their nightmares.

A young woman, who declined to be named, told The Observer: 'I remember the moment the explosion happened, the train was packed as usual, the explosion ripped through the carriage, the blast ripped through the guy in front of me. I would not be here if he was not there; it still has not sunk in what happened. I wonder if he had children, his guts were spilled out. It was worse than anything imaginable.'

Helen Long, a London Underground worker, spent almost two hours with a 25-year-old man called Paul who had lost a leg in the blast. 'We had a joke about the 2012 Olympics - he said he was going to be the first paraplegic athlete to take part,' Long said. Earlier on she had helped the tube train's driver tie his belt around Paul's leg to stem the bleeding. 'That saved his life,' Long said. Just for good measure the three of them said the Lord's Prayer.

First fear, then training kicks in

News of the first explosion reached Scotland Yard within minutes. Sir Ian Blair, the Metropolitan police commissioner, was writing a speech in his eighth floor office about his planned reforms of the service, and he had left it to the last minute: 'I was due on 30 minutes later in front of 500 staff, so I wasn't in the mood to be disturbed.'

A short while earlier, an interview with Blair, recorded the previous evening, had been broadcast on Radio 4's Today programme. Blair had used the interview to talk about the Met's high state of preparedness to deal with a terrorist attack and its heightened state of awareness in the face of very real threats.

Informed of the blast by officers on the ground, Blair's chief of staff, Caroline Murdoch, immediately realised her boss had to be interrupted. 'She popped her head round the door and said, "I'm sorry, I know you don't want to be disturbed, but something has happened you need to know about."'

Early reports were still suggesting the first explosion had been caused by an electrical power surge. 'Like everyone else, my first thoughts were, 'Well, maybe this is a bomb, but maybe it isn't",' Blair said.

Soon, however, news of the second and further detonations was rushed to his office. 'With the second one, it began to feel pretty clear what was really happening.' In such circumstances, Blair said, 'the professional process the whole service had trained for takes over. You call the meetings, appoint the Gold commander, put the emergency plan into action. We had rehearsed for this for a very long time.'

Indeed they had: a tube explosion was one of the scenarios practised in Operation Atlantic Blue, an exercise carried out in April simulating a simultaneous attack in Britain, the US and Canada. Two years ago a major incident was also 'staged' at Bank tube station - chosen because it has the most complicated layout, with multiple exits and several lines criss-crossing each other. It proved invaluable throughout last Thursday.

Just after 9:15am news came through of a third incident on the underground, this time at Edgware Road station in west London. It too was initially blamed on a probable power surge. There was a problem with the network, the authorities said. The tube was going to be shut down while they investigated the cause of the incidents. No need to panic.

Below ground, commuters knew otherwise. The blast, in the first carriage of a tube on a Circle line train leaving Edgware Road for Paddington, had blown through a wall and hit two other trains on an adjoining platform. Initial reports suggested the three explosions on the tube occurred over a 30-minute period, from 8:51 am to 9:17 am. But yesterday police investigators confirmed all three devices were detonated within moments of each other, probably with timers, bringing the tube system to a standstill and terror to the passengers travelling through Edgware Road station.

'I saw one lady who was ripped to pieces, lying between the two trains. People were trying to help her,' said Carol Miller, 35, from Oxford, who was on a Circle Line train travelling in the opposite direction from those hit.

'It was the most horrendous thing I've ever seen in my life. People were screaming out, there was debris everywhere. We were trying to open doors to let in air but we couldn't. There were two tubes crossing in different directions. As it (the other train) got to our carriage it exploded. It was a massive explosion and immediately everything filled up with smoke.'

Survivors were trapped for up to 30 minutes before being led to safety through the tunnel. Above ground, outside the station, the numerous cafes in one of London's most multicultural areas were preparing for brisk business. The genial bustle was shattered by the sound of sirens. Ambulances and police vans descended on the station in their droves. Scores of London Underground workers in red boiler suits and hard hats quickly materialised and sealed the roads off with quiet efficiency.

Then, into the daylight, stumbled the walking wounded. West London's cafe society was transformed into a giant makeshift cottage hospital. Staff at a nearby Marks & Spencer store tried to help passengers offering them bottles of water and tissues.

The local Hilton hotel threw open its doors to take the wounded. A priest was sent in to comfort the traumatised. Double decker buses were used to ferry the injured to nearby St Mary's hospital.

Paul Dadge, 28, a project manager with a knowledge of first aid, helped apply a facial mask to a burns victim. The image was captured on the front page of several newspapers the following day.

'She was called Davina,' Dadge said. 'I believe she was 28. But she was one of a number of people I tried to help. Davina had serious burns to her face and her tights had also been badly burned. She was really brave.'

'It seems unreal. I wish it could be'

It was at 9:47 am the terrible, conclusive proof struck that London was undergoing its biggest attack since the Second World War. The quiet of the capital's leafy Bloomsbury District was shattered by the sound of an enormous blast that ripped the roof off a double-decker bus outside the elegant stucco-fronted headquarters of the British Medical Association (BMA) on Tavistock Square.

The bus, a number 30 travelling from Hackney to Marble Arch, driven by 49-year-old George Psaradakis, was in the area only because it had been diverted due to the earlier incident at King's Cross.

A nurse, who gave only her first name, Josephine, told how she was returning from a hospital night shift. She was sitting on the upper deck of the bus when she heard a huge explosion and was knocked out. When she came round, she saw the man next to her was dead. Josephine had to climb over the man's body to escape the bus. 'I struggled to walk due to the shards of glass in my feet,' she said.