Thousands of people bowed their heads and observed a minute's silence at 11:02 am (0202 GMT), the exact moment of the world's second and last nuclear attack on August 9, 1945, which killed more than 70,000 people.

"We are facing a crisis in terms of the breakdown of the very structure of nuclear non-proliferation," Nagasaki Mayor Tomihisa Taue told the ceremony.

Besides the five established nuclear states of the United States, Russia, Britain, France and China, Taue lashed out at India, Pakistan and North Korea for pursuing nuclear arms "under the excuse of self-defence."

In the Middle East as well, Taue said the "nuclear non-proliferation structure is being shaken" by Israel, which is believed to have atomic weapons, and by Iran, which is suspected of pursuing them.

"The use of nuclear weapons can never be permitted or considered acceptable for any reason whatsoever."

Katsuki Masabayashi, 68, who was 1.3 kilometres (0.8 miles) away from the epicentre of the blast, recalled the horror.

"I dug out my sister, who had fainted, from the rubble. I wandered around carrying my sister on the back," he told the ceremony.

"Bamboo was sticking into the left part of my belly to reveal the flesh while my three-year-old sister, bleeding and shivering with charred clothes on, was calling out 'mama, mama' in a trailing voice.

"That single bomb, which massacred people indiscriminately and instantly with radiation that had an unprecedented destructive force, is the evil of the human race," Masabayashi said, his voice shaking with anger.

The mayor, who took office in April after his predecessor and staunch anti-nuclear campaigner Iccho Ito was gunned down by a gangster, also demanded Japan show "strong leadership" in the non-proliferation drive.

Prime Minister Shinzo Abe pledged his efforts as he stood at the foot of the Peace Statue -- a bronze figure of a man pointing to the sky from which the atomic bomb fell.

"Japan has taken a path of international peace wholeheartedly for the 62 years after the war," Abe said. "Japan, as the only atomic-bombed nation, is responsible to pass down this tragic experience in the international community."

Some of the conservative leader's top aides last year called for Japan to at least study going nuclear after arch-rival North Korea tested an atomic bomb.

The prime minister also faced a backlash in June after his defence minister appeared to justify the nuclear attacks, saying they hastened Japan's surrender and prevented the Soviet Union from seizing large parts of the country.

The minister, Fumio Kyuma, resigned three days after the remarks. Kyuma was not invited to the ceremony, even though he represents Nagasaki in parliament.

Abe, who initially played down Kyuma's remarks as explaining the US position, offered a fresh apology.

"My heart is full of regret as the remark by minister Kyuma hurt the feelings of many victims of the bombings," Abe, who suffered a major election defeat last week, told reporters after the ceremony.

The mayor had charged in his speech that even in Japan there was "erroneous interpretation" of the atomic bombings.

The US nuclear bomb, codenamed "Fat Man" after Winston Churchill, was even larger than "Little Boy" which was dropped three days earlier on Hiroshima, killing some 140,000 people.

Japan surrendered on August 15, ending World War II.

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