"Nuclear weapons are not a necessary evil but an absolute evil, and can never coexist with humanity," Mayor Shiro Suzuki said in a speech broadcast by Japanese public television channel NHK.
The mayor also asked the Japanese government to maintain the "three non-nuclear principles," adopted as a resolution by the Japanese parliament in 1971 but never incorporated into Japanese law: not to possess, not to produce, and not to allow nuclear weapons to be brought into the country.
Japanese Prime Minister Sanae Takaichi said her government defended the three principles without clarifying whether the country would continue to respect them in the future.
She said the government would defend realistic and practical measures to achieve a world free of nuclear weapons.
The Japanese government is preparing a review of its key national security documents that has raised questions about whether the nuclear principles will remain intact, especially after Defence Minister Shinjiro Koizumi recently called for a "taboo-free" national debate on nuclear weapons.
Sunday's ceremony in Nagasaki included a message from UN Secretary-General Antonio Guterres, who made a strong anti-war plea and a case for diplomacy.
"Humanity should not have to live under permanent threat," Guterres said, calling for leadership and political will to reduce risks and move towards nuclear disarmament.
On August 9, 1945, the United States dropped a plutonium-fueled atomic bomb nicknamed Fat Man on Nagasaki, killing about 40,000 people instantly.
Three days earlier, the US had dropped a similar bomb called Little Boy on Hiroshima killing an estimated 80,000 people.
The bombings prompted Japan to announce its surrender to the allied powers on August 15, before officially signing surrender documents on September 2, ending World War II.
