Twin cathedral bells rang in unison on Saturday in Nagasaki for the first time since the atomic bombing of the Japanese city 80 years ago.

On August 9, 1945, at 11.02am, three days after a nuclear attack on Hiroshima, the United States dropped an atomic bomb on Nagasaki.

After heavy downpours on Saturday morning, the rain stopped shortly before a moment of silence and ceremony in which Nagasaki mayor Shiro Suzuki urged the world to "stop armed conflicts immediately".

"Eighty years have passed, and who could have imagined that the world would become like this?," he said.

"A crisis that could threaten the survival of humanity, such as a nuclear war, is looming over each and every one of us living on this planet."

About 74,000 people were killed in the southwestern port city, on top of the 140,000 killed in Hiroshima.

Days later, on August 15, 1945, Japan surrendered, marking the end of World War II.

Nagasaki resident Atsuko Higuchi, 50, said that "instead of thinking that these events belong to the past, we must remember that these are real events that took place.

"In the end... when war breaks out, those who suffer the most are the little children or the mothers raising them."

On Saturday, the two bells of Nagasaki's Immaculate Conception Cathedral rang together for the first time since 1945.

The imposing red-brick cathedral, with its twin bell towers atop a hill, was rebuilt in 1959 after it was almost completely destroyed in the explosion just a few hundred meters away. Only one of its two bells was recovered from the rubble, leaving the northern tower silent.

With funds from US churchgoers, a new bell was constructed and restored to the tower, and chimed on Saturday at the exact moment the bomb was dropped.

The cathedral's chief priest, Kenichi Yamamura, said the bell's restoration "shows the greatness of humanity".

"It's not about forgetting the wounds of the past but recognising them and taking action to repair and rebuild, and in doing so, working together for peace," Yamamura said. (AFP)