Bombers made a second attempt to kill Londoners yesterday, Britain's most senior police officer said last night.

Metropolitan Police Commissioner Sir Ian Blair said the intention of those who tried to set off explosive devices at four locations on the capital's transport network "must have been to kill".

Sir Ian said it was his understanding that some of the devices did not go off.

And he said London Ambulance Service took no casualties to hospital from the scenes.

He refused to go into details about the investigation, or to say whether a hunt for the bombers had begun.

He also refused to confirm suggestions that some of the devices were nail bombs.

Sir Ian confirmed there were four scenes - at the Oval, Warren Street and Shepherd's Bush Underground stations and on a bus in east London - where "attempts have been made to set off explosive devices".

He told a Press conference in London last night: "Clearly the intention must have been to kill. You do not do this with another intention.

"I think the important thing is that the intentions of the terrorists have not been successful."

An arrest at Downing Street and an incident at a hospital yesterday were "entirely unconnected events" to the terrorist attacks, he added.

Sir Ian said he was not yet in a position to say whether yesterday's attacks were connected to the July 7 bombings, which claimed the lives of 56 people.

He also said it was too early to say whether the attacks were the work of the same terror network responsible for the blasts two weeks ago.

But he added: "We do believe that this may represent a significant breakthrough in the sense that there is forensic material at these scenes."

Yesterday's blasts occurred almost simultaneously at about 1pm. Terrified passengers began emerging from the Tube trains and the bus, reporting small explosions and smoke in the carriages.

Some spoke of seeing rucksacks that had detonated, but it quickly became clear that the explosions were not on the scale of July 7.

Independent experts suggested the blasts may have been caused by detonators, not bombs.

Last night, sources said detectives were working on the basis that the bombs were not properly primed.

That could explain witnesses' accounts of suspects fleeing the scenes of some of the blasts, including Warren Street, Shepherd's Bush and the Oval, where there were reports of a man dumping a rucksack in a carriage then fleeing as the doors shut.

Scotland Yard confirmed that armed officers had been deployed to an incident at University College Hospital, near Warren Street.

Three armed officers were seen going into the major incident unit at the hospital, minutes before someone was carried from an ambulance into the unit on a stretcher.

It followed reports that someone had been seen running away from Warren Street after the incident there. Scotland Yard said later that that incident had been stood down.

Three small rooms in an unoccupied part of the hospital remained cordoned off last night.

Stagecoach, which owns the number 26 bus involved in yesterday's incident, said the driver had reported hearing a bang.

The bus had left Waterloo and was in Shoreditch when the incident happened. "The driver heard a bang which appeared to come from the upper deck. When he went upstairs to investigate, the windows on the upper deck were blown out," a spokesman said.

Tony Blair, who was told of the incidents as he met Australian Prime Minister John Howard in Downing Street, urged people to remain calm.

Mr Blair said: "We can't minimise incidents such as these. All I would like to say is this - we know why these things are done, to frighten people and make them anxious and worried.

"Fortunately in this instance there appears to have been no casualties. We have just got to react calmly."

The Prime Minister cancelled a visit to east London yesterday afternoon, but he went ahead with a meeting with senior ministers and leading members of the security and intelligence services.

The meeting had been called to discuss the response to the July 7 attacks and was scheduled before yesterday's incidents.

London Mayor Ken Livingstone also cancelled a visit to the Family Assistance Centre set up to help victims of the July 7 blasts in light of the latest Tube incidents.

Mr Livingstone said last night: "Those people whose memories stretch back to the seventies, eighties and nineties will remember there were horrifying bombing campaigns in London. We got through that and we'll get through this."

Andy Trotter, deputy chief constable of British Transport Police, said anti-terrorist officers were at all four scenes working to recover as much forensic evidence as possible.

Mr Trotter also said no trace of chemical or biological agents had been found at any of the scenes.

* A national freephone hotline for reporting terrorist activity - 0800 789 321 - is being displayed on roundabouts in Hartlepool.

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