Police in London on Saturday said they'd arrested a second man in London in connection with this week's failed transit bomb attacks.

Scotland Yard said the man was arrested in Stockwell, the south London neighbourhood where another suspect was detained on Friday and another man was shot dead by police in a subway station.

The first suspect, who has not been named, was being questioned at a high-security London police station.

Police said the second arrest occurred late on Friday and was "in connection with our inquiries" into Thursday's attacks.

Police are mounting a huge manhunt for the attackers who tried to bomb three subway trains and a bus, two weeks after similar bombings killed 52 people and four suicide bombers. The bombs in the latest attacks failed to detonate fully, and no one was injured.

On Friday, police released closed-circuit TV images of four suspected attackers and appealed to the public for information.