Vowing to never allow a repeat of his city's tragedy, Hiroshima's mayor called on the nuclear powers to abandon their arsenals and stop "jeopardising human survival."

At exactly 8.15am local time, the moment of the blast, the city's trolleys stopped and more than 55,000 people assembled at Peace Memorial Park observed a moment of silence that was broken only by the ringing of a bronze bell.

Then, with offerings of water and flowers for the dead, Hiroshima remembered how the blast turned life to death for more than 140,000 and forever changed the face of war.

Outside the nearby A-Bomb Dome, one of the few buildings left standing after the blast, peace activists held a die-in.

Hiroshima's outspoken mayor, Tadatoshi Akiba, gave an impassioned plea for the abolition of all nuclear weapons, and said the United States, Russia and other members of the nuclear club are "jeopardising human survival".

"Many people around the world have succumbed to the feeling that there is nothing we can do," he said. "Within the UN, nuclear club members use their veto power to override the global majority and pursue their selfish objectives."

In a more subdued speech, the Japanese prime minister, Junichiro Koizumi, offered his condolences to the dead.

"I offer deep prayers from my heart to those who were killed," he said, vowing that Japan would be a leader in the international movement against nuclear proliferation.

Though Hiroshima has risen from the rubble to become a thriving city of 3 million, most of whom were born after the war, the anniversary underscores its ongoing tragedy.

Officials estimate that about 140,000 people were killed instantly or died within a few months after the Enola Gay dropped its deadly payload over the city, which then had a population of about 350,000.

Three days later, another US bomber, Bock's Car, dropped a plutonium bomb on the city of Nagasaki, killing about 80,000 people. Japan surrendered on August 15 1945, bringing the second world war to a close.

Including those initially listed as missing or who died afterward from a loosely defined set of bomb-related ailments, including cancers, Hiroshima officials now put the total number of the dead in this city alone at 242,437.

This year, 5,373 more names were added to the list.

Fumie Yoshida, who survived the blast but lost her father, brother and sister, said she chose not to attend the formal memorial, but joined a small group of friends to pay her respects privately in the peace park. Yoshida was 16 when Hiroshima was bombed.

"My father's remains have never been found," she said. "Those of us who went through this all know that we must never repeat this tragedy. But I think many Japanese today are forgetting."