Speaking before a crowd of survivors, their descendants and visiting dignitaries from around the world, Mayor Matsui urged US President Barack Obama and others to visit his cit, in line with a proposal made at a meeting of the Non-Proliferation and Disarmament Initiative in Hiroshima recently.

"President Obama and all leaders of nuclear-armed nations, please respond to that call by visiting the A-bombed cities as soon as possible to see what happened with your own eyes," Mayor Matsui said.

"If you do, you will be convinced that nuclear weapons are an absolute evil that must no longer be allowed to exist."

Hiroshima launched a campaign this year to send invitations to President Obama, written on paper recycled from tens of millions of origami cranes sent from around the world.

About 45,000 people stood for a minute of silence at the ceremony in Hiroshima's Peace Park near the epicentre of the 1945 bombing that immediately killed up to 140,000 people.

The bombing of Nagasaki three days later killed another 70,000 and an incalculable number have died in the aftermath of the two bombings.

Bells had tolled earlier in the day as ageing survivors, relatives, government officials and foreign delegates observed a moment of silence in the rain at 8.15 am when the detonation turned the city into an inferno.

A US B-29 bomber named Enola Gay dropped the first atomic bomb on Hiroshima on August 6 1945, in one of the final chapters of World War II.

Historians have long been at odds over whether the twin attacks brought a speedier end to the war.

But the bombed cities have bypassed that issue, instead focusing on the central question of whether such weapons should be held, let alone used.

The two cities have spearheaded anti-nuclear movements all over the world, calling atomic bombs "the absolute evil."