Japan marked the 70th anniversary of the atomic bombing of Hiroshima on Thursday.

Japan marked the 70th anniversary of the atomic bombing of Hiroshima on Thursday, with Mayor Kazumi Matsui renewing calls for US President Barack Obama and other world leaders to step up efforts toward making a nuclear-weapons-free world.

Tens of thousands of people stood for a minute of silence at 8:15 a.m. at a ceremony in Hiroshima's peace park near the epicenter of the 1945 attack, marking the moment of the blast. Then dozens of doves were released as a symbol of peace, AP reported.

The U. bomb, "Little Boy," the first nuclear weapon used in war, killed 140,000 people. A second bomb, "Fat Man," dropped over Nagasaki three days later, killed another 70,000, prompting Japan's surrender in World War II.

The US dropped the bombs to avoid what would have been a bloody ground assault on the Japanese mainland, following the fierce battle for Japan's southernmost Okinawan islands, which took 12,520 American lives and an estimated 200,000 Japanese, about half civilians.

Matsui called nuclear weapons "the absolute evil and ultimate inhumanity" that must be abolished, and criticized nuclear powers for keeping them as threats to achieve their national interests. He said the world till bristles with more than 15,000 nuclear weapons.

He renewed an invitation to world leaders to visit Hiroshima and Nagasaki to see the scars themselves, during the G-7 summit in Japan next year.

"President Obama and other policymakers, please come to the A-bombed cities, hear the hibakusha (surviving victims) with your own ears, and encounter the reality of the atomic bombings," he said.

"Surely, you will be impelled to start discussing a legal framework, including a nuclear weapons convention."