Japan on Sunday marked the 70th anniversary of the atomic bombing of Nagasaki that claimed tens of thousands of lives in one of the final chapters of World War II.

Bells tolled as ageing survivors, the relatives of victims and others remembered the devastating blast at 11:02am local time (12:02pm AEST) on August 9, 1945.

About 74,000 people died in the initial blast from a plutonium bomb dubbed “Fat Man”, or from after-effects in the months and years following the bombing.

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The attack on Nagasaki came three days after American B-29 bomber Enola Gay dropped a bomb dubbed “Little Boy” on Hiroshima, the first atomic bombing in history.

Nearly everything around it was incinerated by a wall of heat up to 4,000 degrees Celsius, hot enough to melt steel.

Visitors look at paper lanterns displayed for the Peace Candle Lights at Nagasaki Peace Park in Nagasaki. Photo: AAP

The twin bombings dealt the final blows to imperial Japan, which surrendered on August 15, 1945, bringing an end to World War II.

Japanese prime minister Shinzo Abe laid a wreath of flowers with representatives from 75 countries, including US ambassador Caroline Kennedy, attending the ceremony.

“As the only country attacked with an atomic bomb in war, I am renewing our determination to lead the global effort of nuclear disarmament, to create a world without such weapons,” Mr Abe said in his speech.

He also said the country would continue to abide by its long-held non-nuclear principles: not producing, possessing or allowing nuclear weapons on Japanese territory.

Prime minister Abe was criticised for failing to mention the three principles at the ceremony in Hiroshima, alarming atomic bomb survivors, particularly at a time when the nationalist leader is trying to push through legislation to expand the role of the military.

Nagasaki mayor Tomihisa Taue appeared to implicitly criticise the bills in a speech at the ceremony.

“Worries and anxieties are now spreading among us that this pledge made 70 years ago and the principle for peace in the Japanese constitution may be now undermined,” he said to loud applause.

Japan’s prime minister Shinzo Abe offers a flower wreath for victims on Nagasaki’s 70th anniversary. Photo: AAP

Mr Abe has faced criticism and opposition for his efforts to boost the role of his pacifist country’s Self-Defence Forces, changes that open the door to putting troops into combat for the first time since the war.

A constitution imposed by US occupiers after the war prevented Japan’s military from engaging in combat except in self defence.

Ms Kennedy was accompanied by the US undersecretary for arms control and international security, Rose Gottemoeller.

In a statement, ambassador Kennedy said: “Today as we mourn those who perished in Nagasaki and remember all of the victims of World War II, we recommit ourselves to working toward a world where all people can live in peace.

“In this year which marks 70 years since the end of World War II, the relationship between our two countries stands as a model of the power of reconciliation.

“I would like to thank the citizens of Nagasaki for inviting me to participate in their Peace Memorial Ceremony.”