About 50,000 people gathered Monday in Hiroshima's peace park near the epicenter of the 1945 blast that destroyed most of the city and killed as many as 140,000 people. A second atomic bombing Aug. 9 that year in Nagasaki killed tens of thousands more and prompted Japan to surrender to the World War II Allies.
Prime Minister Yoshihiko Noda said Japan must pass the experience on to future generations so that the lessons of Hiroshima are not forgotten.
Mayor Kazumi Matsui said Japan must lead disarmament efforts. He also said the nuclear accident at Fukushima last year has shown the dangers of nuclear technology, even for peaceful purposes.