At the ceremony on Sunday, the mayor of Nagasaki, Tomihisa Taue, urged world leaders to ban nuclear weapons.
Taue said the threat of the further use of nuclear arms was greater today than ever before.
“[W]e only become aware of the threat of nuclear weapons when they are used again, we are in an irrevocable position,” said Taue.
He urged the Japanese government to sign the Treaty on the Prohibition of Nuclear Weapons.
Japanese Prime Minister Shinzo Abe was in attendance.
On August 9, 1945, a B-29 bomber Bockscar dropped a 4.5-ton plutonium bomb dubbed “Fat Man” on Nagasaki. The bombing resulted in the obliteration of the city and the death of more than 70,000 of its residents.
At 11:02 am local time (0202 GMT), the moment the US atomic bomb hit the city, the attendees at the ceremony, which also included a dwindling number of survivors from the tragedy, held a minute of silence to honor those killed.
Some of the survivors of the US attack developed cancer and other illnesses due to the radioactive fallout that covered the city.
Three days earlier, on August 6, the US had dropped an atomic bomb on Hiroshima. That bombing killed 140,000 people.