The US bomb killed 140,000 people, and a second bombing, over Nagasaki three days later, killed another 70,000.The bombings prompted Japan`s surrender in World War II.
Mayor Kazumi Matsui used the occasion to call on world leaders to step up efforts towards making a nuclear weapons free world.As doves, symbolizing peace, circled above, he called nuclear weapons "the ultimate inhumanity and evil" that must be abolished.
He also criticized nuclear powers for still using the bombs as threats to achieve their national interests."We must establish a broad national security framework that does not rely on use of force but is based on trust," Mr Matsui said.
"Now is the time to take action."
He renewed an invitation to Mr Obama and other world leaders to visit Hiroshima and Nagasaki to see the scars themselves.With the average age of survivors now exceeding 80 years for the first time this year, passing on their stories is considered an urgent task.
Prime Minister Shinzo Abe said that as the sole country to face a nuclear attack, Japan had a duty to push for the elimination of nuclear weapons.The anniversary comes as Japan is divided over Mr Abe`s push to pass unpopular legislation to expand the country`s military role internationally.
Attendants this year included US Ambassador Caroline Kennedy and representatives from more than 100 countries, including Britain, France and Russia.
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