A series of bomb attacks on London's transport network has killed more than 30 people and injured about 700 others.
Three explosions on the Underground left 35 dead, two died in a blast on a bus and another died later in hospital.
Foreign Secretary Jack Straw said the bombings had "the hallmarks of an al-Qaeda-related attack".
Prime Minister Tony Blair promised the "most intense police and security service action to make sure we bring those responsible to justice".
Mr Blair, who had returned to London from the G8 summit in Gleneagles, condemned the terrorists and paid tribute to the stoicism and resilience of the people of London.
"They are trying to use the slaughter of innocent people to cow us, to frighten us out of doing the things that we want to do," he said in a televised statement from Downing Street.
They "should not and they must not succeed," he said.
"We know that these people act in the name of Islam but we also know that the vast and overwhelming majority of Muslims here and abroad are decent and law-abiding people who abhor those who do this every bit as much as we do," he added.
The Queen, who will visit some of those involved in the tragedy on Friday, said she was "deeply shocked" and sent her sympathy to those affected. The union jack flag was flying at half mast over Buckingham Palace.
Further details of the Queen's visit will be announced on Friday morning.
BBC News
