A man has been shot dead by armed officers at Stockwell Tube station, as police hunt four would-be bombers.
Passenger Mark Whitby told BBC News he had seen a man of Asian appearance shot five times by "plain-clothes police officers" with a handgun.
"I saw the gun being fired five times into the guy - he is dead," he said.
Passengers were evacuated from the Northern Line station in south London. The incident followed four attempted bombings in the capital on Thursday.
Police have cordoned off a 200-metre area around Stockwell station.
Services on the Victoria and Northern lines were suspended following a request by the police.
Ambulances, including an air ambulance, have been sent to the scene at Stockwell.
Mr Whitby, told BBC News: "I was sitting on the train reading my paper.
"I heard a load of noise, people saying, 'Get out, get down!'
"I saw an Asian guy run onto the train hotly pursued by three plain-clothes police officers.
'Bomb belt'
"One of them was carrying a black handgun - it looked like an automatic - they pushed him to the floor, bundled on top of him and unloaded five shots into him."
BBC Home affairs correspondent Margaret Gilmore said officers had challenged a known suspect they had been following.
"He ran, they followed him. They say they gave him a warning, they then shot him.
"They brought in the air ambulance. They did everything they can to revive him. He died at the scene."
Police had warned they would shoot to kill if they believed somebody to be a threat, she added.
BBC crime correspondent Neil Bennett said the suspect was being followed as a result of CCTV footage seen by officers investigating Thursday's explosions.
The Muslim Council of Britain said Muslims were concerned about a possible "shoot to kill" policy.
Spokesman Inayat Bunglawala said "There may well be reasons why the police felt it necessary to unload five shots into the man and shoot him dead, but they need to make those reasons clear.
"It's vital the police give a statement about what occurred and explain why the man was shot dead."
Another passenger on the train, Anthony Larkin, told BBC News the man had been wearing a "bomb belt with wires coming out".
"I've seen these police officers shouting, 'Get down, get down!', and I've seen this guy who appears to have a bomb belt and wires coming out.
"People were panicking and I heard shots being fired."
Former Flying Squad officer, John O'Connor, told BBC News: "The man shot must either be one of the bombers or a potential suicide bomber."
Bombers "don't always carry sports bags" and the man could easily have been wearing a bomb belt, he added.
If he had been challenged by police and failed to stop they "have not got a lot of alternative", Mr O'Connor told BBC News.
Officers would be trained to aim for the head as shots to the torso could trigger a hidden explosive device, he added.
Londoner Dan Copeland was in the carriage in which the man was shot.
He told BBC News: "We were sitting for a few minutes on the platform, then we heard shouting from the concourse between the two platforms.
'Dull bangs'
"Then the man burst in through the door to my right and grabbed hold of the pole and a person by the glass partition near the door, diagonally opposite me.
"An officer jumped on the door to my left and screamed, 'Everybody out!'
"People just froze in their seats cowering for a few seconds and then leapt up.
"As I turned out the door onto the platform, I heard four dull bangs.
"I ran past an armed officer who was standing on the platform and ran up the stairs."
After Thursday's London blasts, the bombers fled when detonators went off, causing small blasts, but failed to detonate the bombs themselves.
A massive hunt is under way for the men. Analysts believe evidence from the four scenes - at Oval, Shepherd's Bush, Warren Street and on a bus in Shoreditch in east London - could throw light onto the bomb attacks carried out on 7 July.
On Thursday, it is thought detonators went off, causing small blasts, but failed to detonate the bombs themselves.
The attempted attacks caused travel chaos across the capital, with several lines closed and Tube stations evacuated.
Scotland Yard commissioner Sir Ian Blair said the attacks had been intended to kill people.
That must of been great for them.
. I'm a bit scared, cause I'm still in canada, and in 10 days I'm getting home by plane. But the bad thing is that I'll switch plane in London. I really hope that they'll leave the airports alone. 
