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.

The U.S. 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 U.S. 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."

The anniversary comes as Japan is divided over Prime Minister Shinzo Abe's push to pass unpopular legislation to expand the country's military role internationally, a year after his Cabinet's decision to loosen Japan's war-renouncing constitution by adopting a new interpretation of it.

"We must establish a broad national security framework that does not rely on use of force but is based on trust," Matsui said. He urged the Japanese government to stick with "the pacifism of the Japanese Constitution" to lead the global effort of no proliferation.

Abe, also addressing the ceremony, said that as the sole country to face a nuclear attack, Japan had a duty to push for the elimination of nuclear weapons. He pledged to promote the cause through international conferences to be held in Hiroshima later this month.

With the average age of survivors now exceeding 80 for the first time this year, passing on their stories is considered an urgent task. There were 5,359 hibakusha who died over the past year, bringing the total death toll from the Hiroshima bombing to 297,684.

U.S. Ambassador Caroline Kennedy and representatives from more than 100 countries, including Britain, France and Russia, attended the ceremony.

"Little Boy," dropped from the Enola Gay B-29 bomber, destroyed 90 percent of the city. A "black rain" of radioactive particles followed the blinding blast and fireball, and has been linked to higher rates of cancer and other radiation-related diseases among the survivors.