Makeshift shrines and piles of roses and daisies adorned the front of the Kings Cross rail station, one of the sites of the July 7, 2005, coordinated attacks on three subway trains and a bus. Mourners started laying the first flowers at 8:50 a.m., the exact time of the first attack.
Tim O'Toole, the London Underground managing director, laid a wreath with a note reading, "We will never forget. We shall continue to serve."
A second ceremony began nearly an hour later at Tavistock Square, where the fourth bomb exploded at 9:47 a.m. aboard the No. 30 bus.
Blair attended a private memorial at London's Fire Brigade headquarters during the nationwide silence at noon, marked by the tolling of Big Ben and the ringing of bells at St. Paul's Cathedral.
Play stopped at the Wimbledon tennis tournament. Memorial plaques were unveiled at each of the four Underground stations affected by the attacks. Later Friday, the names of the 52 dead are to be read aloud during an evening service in flower-filled Regent's Park.
"This is a time when our country unites across all races, religions and divides and stands in solidarity with all those who have suffered so much, in sympathy with them and in defense of the values which we share," Blair said.
"It is also an opportunity to reflect on the extraordinary efforts of our emergency services, transport staff, health workers and members of the public, whose courage, professionalism and humanity won the admiration of the world."
The coordinated assault was the first suicide attack in Western Europe as well as the deadliest attack in London since World War II. Within days, detectives identified those they believed responsible — four young men dead in the wreckage. Three were of Pakistani descent and lived in and around the northern English city of Leeds. The fourth was a Jamaican immigrant who settled northwest of London.