The 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 this afternoon "must have been to kill".
He said it was his understanding that some of the devices remained unexploded.
He refused to go into details about the investigation, or to say whether a manhunt was now under way for the bombers responsible and refused to confirm suggestions that some of the devices were nail bombs, and he appealed for "patience".
Sir Ian confirmed there were four scenes - at 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 news conference in central London that "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."
Prime Minister Tony Blair
British Prime Minister Tony Blair urged the public to stay calm. "We know why these things are done. They are done to scare people," he told a news conference. Fortunately, in this instance, there appear to have been no casualties . . . we've got to react calmly."
London's transport network was hit by four attempted bomb attacks in Warren Street, Oval, and Shepherd's Bush train stations. There was also an explosion on the No 26 bus in Hackney.
Police also briefly cordoned off London's University College Hospital after armed officers entered the building.
Police think detonators may have exploded rather than bombs themselves. They said the explosions were not on the same scale as the bomb attacks that killed 54 people two weeks ago today.
London's transport network was evacuated for a time but some lines have begun to reopen.
The small explosion on a bus at the junction of Hackney Road and Colombia Road caused no injuries but blew out windows. It is believed a bang was heard in the upper deck of the bus.
A nail bomb was said to have exploded on a train in Warren Street where passengers reported seeing smoke and one person was hurt. Gunshots were also reported at Warren Street where some people said a man ran away from the scene.
One witness said a passenger on the train near Warren Street station had told him there had been a small explosion in a passenger's rucksack. "The rucksack was blown open by the force of a minor explosion," he said.
The British government's civil contingencies committee - known as Cobra - met in an underground bunker in response to the latest incidents.
A spokesman for the Mayor of London Ken Livingstone said: that he had cancelled all engagements and was in constant contact with the Commissioner of the Metropolitan Police and with the heads of the bus and London Underground services.