Paramedics arrive on the scene in London following an explosion
LONDON, England - 50 are confirmed dead and more than 700 wounded after a series of four "callous" and coordinated bombings in London's transport system, Scotland Yard said.
Witnesses described the horror of seeing victims dying and with serious injuries. There were scenes of panic as power failed on crowded underground trains, and tunnels filled with smoke.
British Prime Minister Tony Blair said it was a "barbaric" terrorist attack as he flew back to London from the G8 summit in Scotland, which he said would go on in defiance of the bombers.
A group, the "Secret Organization group of al Qaeda Organization in Europe," claimed responsibility in a Web site posting. The authenticity of the claim could not immediately be verified.
Police immediately began a hunt for the bombers - thought to have operated similar to the Madrid bomb attackers of March 2004 who left explosives on trains rather than mounted suicide attacks.
The death toll was expected to rise and London hospitals reported many people in critical and serious condition.
Three of the blasts took place in the city's subway system and one more hit a double-decker bus, all at the height of rush hour.
International SOS, an international medical emergency service, reported that the police had found explosive traces in at least one of four confirmed blast locations.
Scotland Yard's Deputy Assistant Commissioner Brian Paddick said there had been no warnings given and the police had received no claim of responsibility for the attacks -- though he later added that police were aware of the Web site claim by a self-claimed al Qaeda-linked group and would be looking at it.
Hospital officials have reported at least 160 wounded. London transit officials shut down the entire Underground and stopped buses in the central city district.
Blair, in Scotland where he is hosting the G8 summit, told reporters he would leave the summit for a "face to face" report in London and then return later in the evening. Blair said it was "also reasonably clear" that the attacks were timed to coincide with the opening of the summit.
The Associated Press and CNN contributed to this story.
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