Nagasaki on Friday marked the 68th anniversary of the atomic bombing by the United States during World War II that turned the Japanese city into an inferno.

Tens of thousands gathered to remember the more than 70,000 people who died instantly in the blast, or of the after-effects in the months and years after the bombing, which hit Nagasaki at 11:02 am local time (0202 GMT).

Bells tolled as ageing survivors, relatives, government officials and foreign delegates observed a moment of silence at the time of detonation.

The bombing of Nagasaki came three days after the first-ever atomic blast at Hiroshima, which claimed about 140,000 lives in all. Hiroshima held its own remembrance ceremonies earlier this week.

"We hold the responsibility to realise a world free of nuclear weapons and pass on to the next generation and to the world the inhumane nature" of atomic weapons, Japanese Prime Minister Shinzo Abe told the ceremony.

An envoy from India, a nuclear power which is not a signatory to the Nuclear Non-Proliferation Treaty, was attending the ceremony for the first time, a Nagasaki official earlier told AFP.

The ceremony is held near the spot where the US military dropped the plutonium bomb, nicknamed "Fat Man", on August 9, 1945, in the final chapter of the war.

US filmmaker Oliver Stone, who was in Japan to attend the remembrance ceremonies, said the widely held explanation that the bombs ended WWII was a "tremendous lie".

"It's easy to look at the issue simply that Americans dropped the bomb to end World War II because Japanese militarists would not give up," the director was quoted as saying.

But "that would be a surface explanation", he was quoted as saying by Kyodo news agency.

Historians have long been at odds over whether the twin attacks brought a speedier end to the war by forcing Japan's surrender and preventing many more casualties in a land invasion planned for later in the year.

Many atomic bomb survivors, known as "hibakusha" in Japanese, oppose both military and civilian use of nuclear power, pointing to the tens of thousands who were killed instantly in the blasts and the many more who later died from radiation sickness and cancer.

Anti-nuclear sentiment has run high in Japan following the 2011 atomic disaster at Fukushima, with most of the country's reactors switched off.