United Nations Secretary-General Ban Ki-moon has called for worldwide nuclear disarmament during a ceremony marking 65 years since the bombing of Hiroshima.
Representatives from the United States, France and Britain attended the annual memorial service in Hiroshima for the first time.
Mr Ban said the anniversary is a timely reminder to world leaders to rid the world of nuclear weapons.
About 140,000 people were killed or died within months of the bomb being dropped by a US B29 aircraft at 8.15am local time on 6 August, 1945.
Japan surrendered after a second atomic bomb was dropped on Nagasaki three days later.