Bombers launched a second attempt to kill Londoners today, Britain's most senior police officer said.
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".
But he said that today the terrorists failed.
Sir Ian said it was his understanding that some of the devices remained unexploded.
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 manhunt was now under way for the bombers responsible.
He also 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 tonight: "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."
Sir Ian said it had clearly been "another difficult day" for the capital.
However, he added: "London has gone past this point again. We are back to business.
"The emergency services of London have done a marvellous job."
Sir Ian said he was not yet in a position to say whether today's attacks were connected to the July 7 bombings which claimed the lives of 56 people.
"I think that will take a little time," he said.
But he added: "There is a resonance here - there were four attacks and there were four attacks before."
Sir Ian added that it was too early to say whether the attacks today were the work of the same terror network responsible for the blasts two weeks ago.
However, he said today's events could provide a significant breakthrough.
"We do believe that this may represent a significant breakthrough in the sense that there is forensic material at these scenes," he said.