A man chased by police ran on to a train. Five shots later, he was dead

All seats were occupied but no one was standing. After the events of the last two weeks, people were understandably tense.

The train, bound for High Barnet, was being held on the platform. The Tube system had earlier been disrupted by a series of false alarms and the delay to their train was not making things any easier for passengers, but at least the doors were open.

No one, however, could have been prepared for the scene that emerged through one set of those open doors moments later.

An Asian man, described as tall, chubby with an almost clean-shaven head, burst into one of the front carriages of the train. According to one witness, he appeared to trip into the carriage. Another suggested he ran into the train and sat down before standing up again seconds later. Unusually in the heat of a July day, he was wearing what appeared to be a padded coat.

Within moments three plain-clothes police officers, one dressed in jeans and carrying what appeared to be Glock pistols, bundled the man to the floor.

Mark Whitby, a 47 -year-old water installation engineer who was en route to meet his boss at London Bridge, was sitting just five yards away. Visibly shaking from what he had seen, Mr Whitby said: "I heard a lot of shouting 'get down, get out.' I saw a chap run on to the train, an Asian guy. He was running so fast he sort of tripped.

"He was being pursued by three guys. They were plain-clothes policemen. One had a black handgun. As he went down, two dropped on to him to hold him down. The other one fired - I heard five shots. I can't tell where they shot him - he was surrounded - possibly to the head."

As passengers fled the platform, crouching and cowering as they ran to safety, paramedics were called to the shot man. He was declared dead where he lay.

The plain-clothes officers were members of elite and secretive Scotland Yard firearms unit. Known as Specialist Firearms Officers (SFO), they are the cream of the SO19 firearms unit, operating in teams of eight to 10 officers in some of the most dangerous police operations.

It was officers from the SFO unit who took part in the operation to foil the attempted diamond heist at the Millennium Dome in 2000. The officers receive extra training from military specialists.

It was later to emerge that the man was not one of the four suspected of carrying out the failed suicide mission in London less than 24 hours earlier, sparking questions as to whether he had been armed or presented an immediate threat. But for those suddenly immersed in the chaos of the station it was no less terrifying. Another passenger, Anthony Larkin, described what he thought was a bomb belt with wires on the dead man.

The shooting was the grim culmination of events that had begun above ground in Stockwell minutes before, linked to police surveillance of a house in the area. Zane Growns, 27, had been above ground. He was standing outside on Clapham Road. What followed was "like a chase in a cop film", he said.

What he saw were up to 20 police officers running past him into the station below. They were carrying machine-guns and handguns. The security presence had been stepped up dramatically overnight in this tough part of inner-city south London following the discovery of a black sweatshirt bearing the legend New York, just half a mile to the east of Stockwell in Cowley Road. The top had been worn by the man police believed to have tried to bomb a Northern line train at the Oval on Thursday.

According to one eyewitness in a block of local authority flats opposite the station, plain-clothes police had challenged a man wearing a blue fleece who promptly fled, leaping over the ticket barriers and down towards the concourse. Police said later he had been challenged and "refused to obey".

Clapham Road is one of the busiest in London. As usual there was gridlock. The traffic lights were on green but the vehicles were snarled up bumper to bumper outside the station. Steven Jones was caught in the jam. "A plain-clothes policeman ran across the front of the car, armed, and into the station," he said. "Another car parked up on the kerb outside the station and three or four officers ran out. On the other side four or five ran in, all armed. There was no dawdling, there was no ambling or jogging, they were properly sprinting into the station. They were following someone or pursuing someone. There was no messing around."

Sitting on the stationary train below was Mr Whitby. He was travelling north on one of the front carriages of the train from his home in Brixton. It was his usual journey into work. Unbeknown to him the man the police were chasing was about to burst into his morning routine, with horrifying consequences.

Mr Whitby said: "As the man got on the train I looked at his face. He looked from left to right, but he basically looked like a cornered rabbit, like a cornered fox. He looked absolutely petrified."

"I'd put him in his mid to late 20s. Quite a chubby build. He had a baseball cap on and a thick padded jacket which I thought was unusual for this time of year."

Within 15 minutes of the shooting, the area above ground was surrounded by the police. Down below, the normally busy platforms had become a crime scene. Witnesses were being interviewed and streets cordoned off.

Later, with Tube services on the Victoria and Northern lines still suspended, forensic officers began their work underground. By the end of the day scaffolding was being erected around the entrance. Another man was later arrested in Stockwell.