The Japanese city of Nagasaki is marking 70 years since the dropping of an atomic bomb by the United States.

A ceremony at the Nagasaki Peace Park observed a minute's silence.

Speeches by a survivor and Nagasaki's mayor both criticised the attending Prime Minister Shinzo Abe for his plans to loosen the restrictions on what Japan's military can do.

At least 70,000 died in the Nagasaki attack, which came three days after another bomb was dropped on Hiroshima.

Nagasaki was only chosen because the original target, Kokura, was obscured by a cloud.

A solemn ceremony in front of guests from 75 countries, including US ambassador Caroline Kennedy, began on Sunday with a declaration read out by children.

A minute's silence and bells marked the time of the explosion at 11:02 (02:02 GMT).

Nagasaki mayor Tomihisa Taue then delivered a peace declaration to the ceremony. He said there was "widespread unease" about Abe's legislation that will alter the constitutional requirement limiting Japan's military to self defence.

A survivor of the Nagasaki attack, 86-year-old Sumiteru Taniguchi, described the injuries he had suffered and said he could not accept Abe's new legislation.

At least 70,000 died in the Nagasaki attack, which came three days after another bomb was dropped on Hiroshima.Pigeons fly around the peace statue during a memorial service for victims of the 1945 atomic bombing, at Nagasaki Peace Park on August 9, 2014. JAPAN OUT AFP PHOTO / JIJI PRESS

In his address to the ceremony, Abe said Japan remained "determined to pursue a world without nuclear weapons".

In a statement read out on his behalf, UN Secretary General Ban Ki-moon said: "Nagasaki must be the last - we cannot allow any future use of nuclear weapons. The humanitarian consequences are too great. No more Nagasakis. No more Hiroshimas."

Analysis: Rupert Wingfield-Hayes in Tokyo

As the peace bell chimed, the people of Nagasaki stopped and bowed their heads remembering that moment 70 years ago when their city was destroyed in a blinding flash of white light.

Nagasaki often gets forgotten as the world focuses on Hiroshima. But the bomb dropped here was made from plutonium and even more powerful.

Perhaps the most powerful moment in the ceremony came when survivor Sumiteru Taniguchi got up to speak. He described his own terrible injuries... of the skin hanging like rags from his arms and back.

But then he turned on Prime Minister Shinzo Abe sitting nearby. Do not meddle with Japan's pacifist constitution, he warned him. The audience erupted in loud clapping.  Abe looked straight ahead, showing no emotion.

'Thunder in a clear sky'

The effects of the bomb were instant and devastating. It destroyed a third of the city, killing thousands instantly and condemning more to death from radiation sickness.

Days later, Japan surrendered, ending World War Two, although the necessity of the two bombs has been debated ever since.

"It was a clear, sunny day and there was a sudden, blinding flash," remembered one Nagasaki survivor, Toru Mine, who now guides visitors at a museum dedicated to the event.

"My first thought was that it should be a thunder, but I soon realised it's bizarre to have a thunder in a clear sky."

Taniguchi still bears scars on his back, the remains of three ribs that half rotted after the bomb dropped protruding from his chest.

''While people around me were dying, I lived. People say I survived but I think I was kept alive. I am still suffering," he said.