More bombings in London?

Bombers have again targeted London's transport system - with up to four explosions reported.

Metropolitan Police Commissioner Sir Ian Blair said the bombs appeared to be smaller than used in the attacks two weeks ago but advised people to "stay where you are".

Passengers reported one blast at Warren Street station off Tottenham Court Road in central London.

There has also been an incident on a bus in Hackney, East London.

Stations at Warren Street, Oval and Shepherd's Bush have been closed.

Scotland Yard have confirmed there is an incident involving armed police officers at University College Hospital in Bloomsbury close to Warren Street station.

A man was also arrested by armed police at the gates of Downing Street.

UPDATE: According to the San Jose Mercury News, one person was wounded. Apparently no one was killed, which is a relief.

UPDATE: More from London's Daily Mail, which reports that it may have been only the detonators which went off, but that there also may have been "a shooting":

Earlier, Police sources have said that the explosions may have been detonators rather than bombs themselves.

An initial examination carried out by police in protective clothing at Oval station has shown "no trace of chemical agents", a spokesman for Scotland Yard said.

A British Transport Police spokesman said: "One person has received an injury at Warren Street.

"We cannot confirm what the injury is, how it was received or who serious it is. We are still waiting for more information."

There was also a report of a "shooting" at a station.

Metropolitan Police Commissioner Ian Blair told Londoners to "stay where you are" and advised against commuters rushing to rail stations.

MORE: The Daily Mail also has this report that a man "tried to flee after rucksack exploded" at Oval:

An eyewitness who was on the train at Oval station described seeing a man flee after his rucksack exploded, with other passengers trying to stop him.

She told Sky News: "There was a woman with a baby and there was a man standing beside her with a rucksack.

"There was a little explosion. As soon as the door opened the man ran away and people were trying to run after him. There were three men struggling with him but he ran off and they couldn't catch him."

MORE: In the course of research, I saw that Charles Johnson beat the LA Times to the story. (And the LA Times is a prestigious Google news site!)