Bombers launched a second attempt to kill Londoners yesterday - exactly two weeks after a series of suicide bombings which claimed 56 lives in the capital.
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".
He said it was his understanding that some of the devices remained unexploded, and he added 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".
But sources last night said detectives were working on the basis that the bombs were not properly primed.
That could explain eyewitness accounts of suspects fleeing the scenes of at least some of the blasts.
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 said: "Clearly the intention must have been to kill. I think the important thing is that the intentions of the terrorists have not been successful."
He said he was could not say whether the attacks were connected to the July 7 bombings.
Andy Trotter, Deputy Chief Constable of British Transport Police, said no trace of chemical or biological agents had been been found at any of the scenes.
Commenting on the safety of travelling on public transport in London, he said: "This is a huge challenge and we cannot pretend it is not.
"But we are not going to have airline-type security on the buses, trains or Tube - that would bring things to a halt."
Instead, there would be more CCTV, random searches and an increase in the number of uniformed and plain clothes police officers, he said.
The blasts occurred almost simultaneously around 1pm. Terrified passengers began emerging from the Tube trains and the bus, reporting small explosions and smoke in the carriages.
David Walker, a 42-year-old PR manager from Redditch, Worcestershire, was close to Warren Street Tube station in the aftermath of the explosion there.
He was making his way back to Euston station after a business meeting when people emerged from the Underground.
"There was a sense of deja vu about it almost," he said. "There was no panic, more a sense of incredulity. Word was getting round there had been three explosions on the Tube and one on a bus.
"It seemed a carbon copy of a fortnight ago. We didn't know if it had been a full-blown terrorist attack or a copycat thing. It was unnerving but there was almost a stoic British resolve about it."
He added: "I think most people were just glad to be able to get on their mobiles and tell their family they were okay."