Four bombs were planted and at least two exploded on London transport early this afternoon. There are also unconfirmed reports that a bomber was cornered and arrested in University College Hospital.

Another man of Asian appearance was arrested near Downing Street, but police have not confirmed his involvement.

One person is reported injured in the attacks that have been described by Police Commissioner Sir Ian Blair as "smaller than before", but still a "very serious incident"

A nail bomb exploded on a Tube train at Warren Street, where eye witnesses also reported seeing a young Asian man dump a rucksack and run from the scene.

A second bomb exploded on the top deck of the No.26 bus on Colombia Road in Bethnal Green, blowing out the windows.

Bombs were also planted at Oval and above ground at one of the Shepherds Bush stations, which have also been evacuated and cordoned off.

Armed police and dogs have been deployed at University College Hospital, near Warren Street.

Three armed offices were seen going into the hospital, and there have been unconfirmed reports of a man with wires protruding from his jacket being sought among the hospital buildings.

The situation remains fluid and police are asking Londoners not to travel.

Police are investigating the possibility that the injured person at Warren Street might have planted the bomb.

Though the No.26 bus had its upper deck windows blown out, no one was hurt.

A "Code Amber" alert has been declared on the Underground, however the majority of lines are still running.

London Underground (LU) quickly suspended the Victoria, Northern and Hammermith and City lines, however the Victoria Line reopened either side of Warren Street shortly after 3.00pm.

In an apparent security alert the Bakerloo Line and Piccadilly lines were closed mid-afternoon.

Westminster and Waterloo Tube stations were also closed.

Early witness reports maintained that shots had been fired, though the possibility is being investigated that these were exploding detonators that failed to set off their bombs.

Emergency services have been sent to all three Tube stations and the bus scene, Scotland Yard said.

Eye witnesses at Warren Street reported that the bomb went off in a train just as it was approaching the station.

As the train continued running passengers started pouring through the train to get away from the carriage in which the bomb had gone off.

One passenger pulled the emergency cord, stopping the train and opening the doors.

Passengers then flooded the platform and in the resultant confusion panic broke out.

Some witnesses have reported seeing the explosion itself, others that a young Asian man was actually pursued out of the station.

The London Amublance Service (LAS) reported sending four three ambulances to The Oval at 12.38pm, and five to Warren Street at 12.45pm.

The Prime Minister cancelled his afternoon engagements and convened a meeting of the government's COBRA emergency group.

Read more about this story at www.thisislocallondon.co.uk.