The Japanese city of Hiroshima has marked the anniversary of its destruction by the world's first atomic bomb 60 years ago.
About 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 was dropped by a US plane, 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.
Besides those who were killed instantly, many died later from severe burns or radiation.
There is continuing controversy over whether the bomb constituted a war crime, but many commentators believe the US 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. As dusk fell, paper lanterns were floated down a river by the park to represent the souls of the dead.
Japanese 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 minute of silence, 60 years on from the instant the blast struck the city.
A huge metal bell tolled in memory of the victims.
Mr Akiba warned nuclear powers that they were "jeopardising human survival" by clinging on to their arsenals.
"We have to pay due tribute to all the souls claimed by the atomic bomb," he said. "We will not make the same mistake again."
The speaker of Japan's parliament, Yohei Kono, said militarism had led Japan to disaster in World War II.
Fumie Yoshida, who survived the Hiroshima blast aged 16 but lost her father, brother and sister, said she had paid her respects privately.
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."
THE ATTACK ON HIROSHIMA
0812 local time, 6 August 1945:
1. American B-29 bomber 'Enola Gay' approaches Hiroshima at an altitude of about 9,357 metres, and begins its bombing run
2. At 0815 it releases the atomic bomb 'Little Boy'
3. The aircraft then performs a sharp, 155 degree right turn and dives an estimated 518 metres
4. The bomb explodes with a force of 13 kilotons at a height of approximately 576 metres above the city
5. About a minute later the first shock wave, travelling at about 335 metres per second, hits the aircraft