More than half a dozen explosions rocked the London subway and tore open a packed double-decker bus during the morning rush hour Thursday.
The blasts killed at least 50 people and injured scores in what a shaken Prime Minister Tony Blair said was a series of "barbaric" terrorist attacks.
Blair said it was clear the attacks were designed to coincide with the opening of the G-8 summit in Gleneagles, Scotland.
He said the meeting of world leaders would continue but that he would return to London.
"Whatever they do, it is our determination that they will never succeed in destroying what we hold dear in this country and in other civilised nations throughout the world," Blair said a day after London was awarded the 2012 Olympics.
Bloodied and bandaged witnesses reported panicked crowds fleeing the blast sites. A witness at the bus explosion said the entire top deck of the bus was destroyed.
Sir Ian Blair, London's police chief, said he was concerned the explosions were a coordinated attack but said he wouldn't speculate on who was responsible. He said officials had found indications of explosives at one of the sites.
A senior Israeli official said Scotland Yard told Israel minutes before the explosions that it had received warnings of possible terror attacks.
One witness, Darren Hall, said some passengers emerging from an evacuated subway station had soot and blood on their faces. He told BBC TV that he was evacuated along with others near the major King's Cross station and only afterward heard a blast.
Police confirmed an explosion destroyed a double-decker bus at Russell Square in central London. Dow Jones Newswires reported that police said there were explosions on two other buses.
"I was on the bus in front and heard an incredible bang, I turned round and half the double decker bus was in the air," Belinda Seabrook told Press Association, the British news agency.
She said the bus was packed with people. "It was a massive explosion and there were papers and half a bus flying through the air," she said.
Police said incidents were reported at the Aldgate station near the Liverpool Street railway terminal, Edgware Road and King's Cross in north London, Old Street in the financial district and Russell Square in central London, near the British Museum.
Bradley Anderson, a subway passenger, told Sky News that "there was some kind of explosion or something" as his train reached the Edgware Road station in northeast London.
"Everything went black and we collided into some kind of oncoming train," Anderson said.
Simon Corvett, 26, who was on an eastbound train from Edgware Road station, said: "All of sudden there was this massive huge bang."
"It was absolutely deafening and all the windows shattered," he said. "There were just loads of people screaming and the carriages filled with smoke.
"You could see the carriage opposite was completely gutted," he said. "There were some people in real trouble."
London's cell phone network was working after the explosions but was overloaded and spotty, limiting communication.
The explosions sent stocks plummeting in Europe, with several of the major indices down 3 percent.