Metropolitan police commissioner Sir Ian Blair said Scotland Yard officers are involved in the "greatest operational challenge" the force has ever seen, as they seek to find those responsible for the recent attacks on the capital.
Images of four men police want to question in connection with yesterday's failed bomb attacks on London's tube network have been released.
As part of what has been described as a "fast-moving" inquiry, armed police shot dead a man on a tube train in Stockwell at about 10:00 BST this morning in an incident that was "directly linked" with yesterday's failed explosions on a bus and three tube trains.
The man was challenged but refused to obey police instructions, Sir Ian said, which led officers to open fire. Ambulance services were called but he was pronounced dead at the scene.
Properties in central London and west Kilburn have been raided by police and a man was reportedly arrested outside Canary Wharf tube station on a day of intense police activity.
At a press briefing this afternoon, police confirmed that they "urgently want to trace" four men in relation to "attempting to detonate explosives" yesterday, and asked members of the public to contact them with any information.
If anyone sees the men, they should ring 999 and request immediate police assistance. If they know their whereabouts they should ring the anti-terrorist hotline on 0800 789 321.
One image shows a man in a New York embroidered top running away from Oval tube station after a bag was left at the scene.
Another shows a man with a backpack at Westbourne Park tube station on his way to Shepherd's Bush, where another bag was left.
A third image shows a man in a baseball cap on the top deck of the number 26 bus on Hackney Road, where an explosion occurred yesterday.
And a fourth is of a black man at Warren Street station, where another bag containing explosives was found.
Police say that all four are suspected of being responsible for the "partially detonated" bombs at the four sites.
The bombs were said to be homemade explosives and were contained in dark coloured bags, but police say that it is too early to tell how they were detonated.
Metropolitan police assistant commissioner Andy Hayman said that yesterday's attempted attacks "bear similarities" with those on the capital's transport network on July 7th, which killed 56 people.