Millions of Britons stopped what they were doing today to observe two minutes of silence in memory of the victims of last week's suicide bombings in London, a commemoration that was also marked elsewhere in Europe.

The tribute came as British antiterrorism investigators pressed their probe of the July 7 bombings, pursuing possible accomplices of the four attackers and asking the public for information on one man in particular: an 18-year-old British citizen of Pakistani origin who police say set off a bomb on a double-decker bus.

Police released a photo of the suspected bus bomber, Hasib Mir Hussain, who was reported missing by his family on the day of the bombings. His identification documents were found in the wreckage of the red London-style bus after it blew up at Tavistock Square, killing 13 people.

Authorities today raised the total death toll in the four bombings to 53, including the four suicide bombers. More than 700 people were reported injured in the blasts, which struck three subway trains in addition to the bus.

In a televised news briefing, Peter Clarke, head of Scotland Yard's antiterrorism branch, asked the public for help in tracing the movements of Hussain and urged people who may have seen him board the No. 30 bus to call a hotline.

"We're also keen to speak to everyone on the bus," he said. He asked people who were among the estimated 80 passengers on the bus to call police if they had not already been interviewed.

In a separate briefing, Metropolitan Police Commissioner Ian Blair confirmed officially for the first time that the attackers were suicide bombers.

"We are as certain as we can be that four people were killed and they were the four people carrying bombs," Blair told the Foreign Press Association. He said police believe they know the identities of all four.

Three of the bombers have been identified in news reports as Britons of Pakistani origin: Mohammad Sidique Khan, 30, Shahzad Tanweer, 22, and Hussain. The fourth has been named by British news media as Lindsey Germaine, a Jamaican-born resident of Britain.

On Wednesday night, police and forensics experts raided a house in Aylesbury, about 40 miles northwest of London, where Germaine reportedly lived.

Police also searched another home in Leeds, the city in northern England that was home to at least two of the suicide bombers. Authorities had raided six homes in and around Leeds on Tuesday, arresting a relative of one of the suspects.

There were also reports that police were looking for at least two other people, including an Egyptian-born chemistry professor who formerly studied at North Carolina State University.

Blair, the London police commissioner, refused to say how many other suspects are being sought.

"We don't know if there is a fifth man, or a sixth man, a seventh man," he said, according to the Associated Press.

At the stroke of noon, traffic in London stopped and people fell silent in tribute to the bombing victims as the chimes of Big Ben marked the two-minute commemoration.

Takeoffs and landings at London's Heathrow airport were briefly suspended, buses and taxis pulled over on city streets, and people stopped and bowed their heads.

Play at the British Open golf tournament in Scotland was also halted for two minutes, as players observed the silence. At the 14th hole, Tiger Woods doffed his cap, closed his eyes and bowed his head.

Outside Buckingham Palace, Queen Elizabeth II stood still during the tribute, as did a crowd that filled Trafalgar Square.

Prime Minister Tony Blair interrupted a reception he was hosting at his residence at No. 10 Downing Street to participate in the commemoration while standing in his garden. Members of the British Parliament suspended debates to join in the silence.

In Paris, sirens wailed and French President Jacques Chirac stood silently at attention outside the Elysee Palace with the visiting Brazilian president.

In Madrid, where 191 people were killed in terrorist train bombings last year, Mayor Alberto Ruiz Gallardon joined other officials in observing the silence in a plaza outside the town hall, AP reported.