A police spokeswoman said the man was challenged by officers at Stockwell station and then shot and killed.
Subway passenger Mark Whitby was an eyewitness to the shooting and described to Reuters what he saw.
"I heard a lot of shouting, 'Get down, get out.' I looked to my right. I saw a [man] run onto the train. Asian guy," Whitby said. "He ran onto the train, he was running so fast he half, sort of, tripped. He was being pursued by three guys. One had a black handgun in his hand, left hand, and as [the man being pursued] sort of went down, two of them sort of dropped onto him to hold him down and the other one fired. I heard five shots."
The shooting follows four nearly simultaneous bombings yesterday that targeted three underground stations and a double-decker bus. The devices detonated without causing any injuries. Two weeks ago, four suicide bombers struck London's transportation system, killing more than 50 people.
The anxiety in the "tube" -- as London's metro system is known -- is palpable. Prime Minister Tony Blair is calling on people to stay calm and get on with their lives as normal; but the trepidation is there, in their eyes.
For all the fear, though, yesterday's detonations might have played into the hands of the security services investigating the London bombings of 7 July. They now have new possible clues in their search for the bombers.
"If they have the devices intact, and if they capture the individuals, it will be incredibly valuable in providing information about other possible plans for attacks and other possible bombers," said Colonel Christopher Langton, the head of the Defense Analysis Department at the International Institute for Strategic Studies in London.
It might have been this that put police on the trail of the man shot dead at Stockwell station.
The similarities between the bomb attacks of 7 July and yesterday are uncanny. On each occasion, four bombs were planted -- three on underground trains and another on a bus. In each case, rucksacks were used to carry the bombs. And in each case, the explosions were closely synchronized.
But yesterday's bombers failed to create the same havoc. Was this deliberate -- merely an attempt to sow panic? Was a copycat group involved? Or did the bombers simply fail in their task?
Langton said he thinks they just got it wrong.
"We're probably looking at explosives that did not ignite after detonation," Langton said. "It could be a copycat, but if it was copycat, we're faced with thinking of people who have detonators and equipment in their possession. And not many people, frankly, could get their hands on that sort of equipment. So my feeling is, this was a serious attempt."
Crispin Black, a former British intelligence officer, told the BBC that the explosives in yesterday's bombs might have belonged to the same batch used in the deadly 7 July attacks and simply degraded over time.
"I suspect it may have been low-grade explosive, badly constructed devices, or something of that nature, indicating that possibly the original technician might have died in the first attack and was not available for the second attack," Langton agreed, "and this was perhaps a lesser technician who put these bombs together."
Meanwhile, a statement posted today on an Islamic website in the name of an Al-Qaeda-linked group, the Abu Hafs Al-Masri Brigade, claims responsibility for both series of attacks. The statement's authenticity cannot be verified.
Although the bombers appear to have failed in their original intent, the psychological damage they have caused to public morale is enormous. However reassured the British public might be by the prompt response of the police to the latest attacks, the fact remains that the bombers got through again.
There was no warning, no advance information. Only a technical failure saved London from further carnage.
"Britain Searches For Responses To London Bombings"