The Japanese city of Hiroshima has marked the anniversary of the moment an atomic bomb exploded above the city 60 years ago. Around 140,000 people were killed by the bomb and its aftermath.

Nuclear survivors, known as Hibakusha, joined dignitaries at the annual commemoration in the Peace Park, built at the epicentre of the blast.

The head of the UN has said the world has made little progress in tackling the spread of nuclear weapons. "Today, we are all Hibakusha," Kofi Annan said in a statement read out on his behalf at the Hiroshima ceremony. He called for concerted action to prevent "a cascade of nuclear proliferation".

Some 55,000 people thronged into the peace park to remember the moment the bomb exploded in the skies above the city, at 0815 on the morning of 6 August, 1945.

Nicknamed "Little Boy", it generated a wave of heat which reached 4,000C (7,200F) and expanded across a radius of 4.5km (2.8 miles), obliterating the city.

Thousands were killed instantly and many others died later from severe burns or radiation. Many commentators believe the attack helped bring an early end to World War II in the Pacific.

During the ceremony, children dressed in black and white, the colours of mourning, laid wreathes of flowers at a simple, arch-shaped memorial. Ladles of water were also offered for those who suffered the atomic heat.

Japan's Prime Minister, Junichiro Koizumi, said that, after the bomb, the city had relentlessly pursued peace. "The citizens of Hiroshima are the witnesses of global peace, we hope that Hiroshima will continue to be the symbol of global peace," he said.

Hiroshima's mayor, Tadatoshi Akiba, led the crowd in a moment of silence, 60 years on from the instant the blast struck the city. A huge metal bell tolled in memory of the victims. He said respect for victims of war demanded abolition of all nuclear weapons. "We have to pay due tribute to all the souls claimed by the atomic bomb," Mr Akiba said. "We will not make the same mistake again". [News report from BBC NEWS]