Japan has marked 64 years since Hiroshima was hit in the world's first atomic bomb attack with a call for a nuclear-weapons-free world.

The mayor of Hiroshima, the city where 140,000 people died from the blast, renewed his call for the abolition of what he said are 24,000 remaining nuclear warheads over the next decade as he led the solemn dawn ceremony.

About 50,000 people, including 'hibakusha' or atom bomb survivors, politicians and envoys from 59 countries and the UN, gathered near the A-bomb Dome, the skeleton of a hall burned by the bomb's intense heat.

'The abolition of nuclear weapons is the will not only of the hibakusha but also of the vast majority of people and nations on this planet,' said mayor Tadatoshi Akiba.

Mr Akiba praised US President Barack Obama for stating at a speech in Prague this year that the US, as the only country to have ever used an atomic weapon, has 'a moral responsibility' to work toward their eventual abolition.

'We refer to ourselves, the great global majority, as the 'Obamajority,' and we call on the rest of the world to join forces with us to eliminate all nuclear weapons by 2020,' Mr Akiba said.

Those at the memorial ceremony offered their silent prayers at 8.15am (12.15 am Irish time), the exact moment the bomb was dropped in 1945.

The blinding blast of 'Little Boy' and the fallout from its mushroom cloud killed 140,000 people, either instantly or in the days and weeks that followed as radiation or horrific burns took their toll.

Three days after the attack, the US dropped a second nuclear bomb on Nagasaki, which killed 70,000 people in the southern port city.

Japan's Prime Minister Taro Aso told the ceremony that 'I pledge anew today that Japan will be the frontrunner in the international community in abolishing nuclear weapons and realising eternal peace.'