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There were three explosions on the Underground - which police said left 33 dead - and one on a double-decker bus in which an unknown number died.
UK Prime Minister Tony Blair, who has returned to London from the G8 summit, has described the attack as "barbaric".
An Islamist website has posted a statement - purportedly from al-Qaeda - claiming it was behind the attacks.
US President George Bush told reporters at the G8 Summit in Gleneagles that "the war on terror goes on."
The Queen said she was "deeply shocked" and sent her sympathy to those affected.
Home Secretary Charles Clarke said the blasts occurred between Aldgate East and Liverpool Street tube stations; between Russell Square and King's Cross tube stations; at Edgware Road tube station; and on a bus at Tavistock Square.
"We will not yield to these terrorists, we will find them, we will bring them to justice," he said.
Paul Woodrow, of the London Ambulance Service, is in Russell Square near the bus which exploded.
"At King's Cross station there is a rescue operation in the tunnel down on the line," he said.
"Although we cannot confirm casualties - it is too early - we are dealing with large numbers of casualties."
Metropolitan Police Deputy Assistant Commissioner Brian Paddick confirmed 33 people had died in the blasts on the Underground. He said it was not yet known how many died in the bus blast.
One caller to BBC Five Live said his friend had seen "the bus ripped open like a can of sardines".
Cnet is reporting that over 45 dead.
In related news at BBC, London mobile networks jammed.