Authorities in London now say 37 people were killed and 700 injured in the bombs that ripped through three subway trains and a double-decker bus Thursday.
British Prime Minister Tony Blair says the terrorists behind the attacks were acting in what he calls "the name of Islam."
Mr. Blair, who spoke after returning to London from a Group of Eight summit in Scotland, said most Muslims around the world deplore such acts.
He pledged to bring the perpetrators to justice, and said Britain will not be intimidated. He called today a very sad day for the British people.
The attacks occurred at the height of morning rush hour. Police say they received no prior warning.
A group calling itself the Secret Organization of al-Qaida in Europe posted a statement on an Arabic website taking responsibility. The authenticity of the statement could not be confirmed.
Information for this report is provided by AP and AFP.