http://news.bbc.co.uk/1/hi/uk/4659093.stmTwo people have been killed and scores have been injured after three blasts on the Underground network and another on a double-decker bus in London.
UK Prime Minister Tony Blair said it was "reasonably clear" there had been a series of terrorist attacks.
He said it was "particularly barbaric" that it was timed to coincide with the G8 summit. He is returning to London.
An Islamist website has posted a statement - purportedly from al-Qaeda - claiming it was behind the attacks.
Home Secretary Charles Clarke said 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.
The Queen said she was "deeply shocked" and sent her sympathy to those affected.
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, and we believe there are a number of casualties."
London's police chief Sir Ian Blair said there had been "many casualties" but it was too early to give a precise figure.
It's particularly barbaric that this has happened on a day when people are meeting to try to help the problems of poverty and Africa
Tony Blair
Blair statement in full
More than 182 casualties and one of the dead were taken to the Royal London Hospital in Whitechapel but it said that 123 people had since been released.
And St Mary's Hospital said it was dealing with 26 injured people, including four with critical injuries and eight in a serious condition.
Sir Ian urged people to stay where they were and not to call emergency services unless it was a life-threatening situation.
He reassured the public that an emergency plan was in place and the situation was "steadily coming under control".
London Mayor Ken Livingstone, speaking from Singapore before flying back to the UK, said Londoners would not be divided by a "cowardly attack."
He said it was an "indiscriminate" attempt at slaughter with no consideration for age or religion.