Updated 2.10am: FOUR bombs were planted and at least two exploded on London transport early this afternoon.

One person may have been injured in the attacks that were described by Met police chief Sir Ian Blair as "smaller than before", but still a "very serious incident".

At around 12.45pm a device exploded on a Tube train at Warren Street, where eye witnesses reported seeing a young Asian man dump a rucksack and run from the scene.

There are also unconfirmed reports that a bomber was cornered and arrested after fleeing into University College Hospital, less than 100 yards away.

Eyewitnesses said at least one man was cornered by armed police inside the hospital and another may have been hiding in a cupboard.

About 45 minutes later a second bomb went off on the top deck of the No.26 bus in Hackney Road, Bethnal Green, blowing out the windows.

At the same time bombs were also planted at Oval and above ground at one of the Shepherd's Bush stations.

All three stations, along with Euston, have been evacuated and cordoned off.

"Clearly the intention must have been to kill," Sir Ian said at a press conference late afternoon. "I mean, you don't do this with any other intention."

The commissioner added: "The important point is that the intention of the terrorists has not been fulfilled."

Like the bombings of July 7, which killed 56 people, the attacks took place at around the same time - first on three Tube trains and then on a bus. As before, all three Tube lines run through King's Cross.

Sir Ian said the latest attacks had "some resonance" with the terror of two weeks ago. Yet it would take "just a little bit longer" to say whether the same group of people were behind it.

Two men of Asian appearance were arrested in Whitehall, one near Downing Street, but police have not confirmed their involvement.

The man detained at Whitehall was later released.

Near Warren Street three armed officers in flak jackets and two sniffer dogs were seen going into University College Hospital.

A short while later a marked Shogun 4x4, escorted by police motorcycles, left the scene. Detainees are often transported in this way.

According to unconfirmed reports, a man with wires protruding from his jacket was sought and arrested.

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

Eye witnesses at the central London station reported that the bomb went off in a train just as it was approaching the platform.

As the train continued running, commuters poured along 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.

People then flooded the platform. Panic broke out.

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

Witnesses at Shepherds Bush reported a man had threatened to blow himself up before fleeing the scene.

In Hackney Road the bomb-hit route 26 bus stood at a bus stop with its hazards still flickering. The explosion shattered some of the windows, but no-one was injured.

London Amublance Service (LAS) reported sending four ambulances to The Oval at 12.38pm, and five to Warren Street at 12.45pm. They took no casualties from the scenes.

Sir Ian said one casualty was reported at one hospital. It "may have been self reporting and may or may not be connected to this", he added.

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

In a press conference Mr Blair said the prupose of the attackers was "precisely to make people worried and frightened". He urged people to stay clam and keep reacting in the way they do.

London mayor Ken Livingstone stressed the need for "continued vigilance" by all Londoners.

"Our safety rests on all Londoners and all London's communities standing and acting together against this common threat."