LONDON — London Mayor Ken Livingstone laid a wreath outside the city's Kings Cross rail station today to commemorate the moment a year ago when the first suicide bombers in western Europe killed 52 people, along with themselves, in coordinated attacks on the transit system.

A pile of yellow roses and daisies lay beneath a sapling in the memorial garden in front of the station. Tim O'Toole, the London Underground managing director, laid a wreath of flowers with a small note reading, "We will never forget. We shall continue to serve."

London has made a determined effort to return to normal life, but a sense of mourning descended on the city one year after the deadliest attack on the capital since World War II.

The buses and subway cars were standing-room only during the morning rush hour — a sign of how last year's carnage in London's transit system has not disrupted life in the capital. But the atmosphere on the underground was tense and subdued, as the city was reminded of the 52 people who, on July 7, 2005, never made it to their destination.

At 8:50 a.m. — a year to the minute after three of the four suicide bombers detonated their deadly cargo — the first flowers were laid in front of King's Cross station, the first public expression of sorrow on a day filled with ceremonies of remembrance.

A second ceremony began nearly an hour later at Tavistock Square, where a fourth bomb detonated at 9:47 a.m. The image of the destroyed No. 30 bus, sitting in the middle of the picturesque London square, became one of the iconic images of the day.

"It's so strange, because I can remember exactly what I was doing this time last year. Everything was all normal and then suddenly, it was not. People were going about their normal daily business and then bombs were going off," said Angelina Alcorn, 26, a nurse who works at the nearby University College Hospital and who helped many of the injured. "I will never forget the image of that bus. It is stuck in my mind."

The bus driver laid a large wreath of pink and white carnations, roses and lilies. On the card, he had written, "You will never be forgotten. May you rest in peace. George Psaradakis. No. 30 bus driver."

Memorial plaques were unveiled at each of the four Underground stations affected by the attacks. A national, two-minute silence is planned for noon, and the names of the 52 dead are to be read aloud during an evening service in flower-filled Regent's Park.

On the eve of the anniversary, a chilling video of one of the bombers threatening more terror was broadcast by Al-Jazeera. It was unclear how the video was obtained or how soon it was made before the attacks.

"What you have witnessed now is only the beginning of a series of attacks that will continue and increase in strength," said Shehzad Tanweer, 22, whose backpack bomb killed six people and himself aboard a Circle Line subway in east London.