Tens of thousands bowed their heads at a ceremony in the Japanese city of Hiroshima yesterday, the 63rd anniversary of the world's first atomic attack.

A bell tolled at 8:15 a.m. to mark the exact moment when the bomb dubbed "Little Boy" was dropped on the city, killing tens of thousands immediately and many more later from radiation sickness.

"We who seek the abolition of nuclear weapons are the majority," mayor Tadatoshi Akiba said in a speech at the Peace Memorial Park, attended by Prime Minister Yasuo Fukuda and elderly survivors of the attack.

"Last year 170 countries voted in favor of Japan's UN resolution calling for the abolition of nuclear weapons. Only three countries, the United States among them, opposed this resolution," he said.

The United States and other world powers fear Iran is developing nuclear weapons, while Teheran says its atomic program is for power generation. Washington and others have warned of more sanctions against Teheran, who they accuse of playing for time in the dispute.

The mayor of Hiroshima also vowed to do more to help the survivors still suffering the physical and mental after-effects of the 1945 attack by the United States in the final days of World War Two, which was followed a few days later by a nuclear attack on the southern Japanese city of Nagasaki.

"We must not repeat such a sad event," one mother attending the ceremony told broadcaster NHK. "We need to pass this message on to our children's generation."

An estimated 140,000 people were killed instantly or died within a few months after the American B-29 bomber Enola Gay dropping its deadly payload. Japan's official death toll of nearly 260,000 includes injured who have died in the decades since.