A ceremony on Saturday to mark the anniversary of the Hiroshima bombing was dominated by national soul-searching on atomic power as Japan's prime minister pledged a nuclear-free future.

Marking the 66th anniversary of the world's first atomic bombing at an annual event usually devoted to opposing nuclear weapons, Prime Minister Naoto Kan said the ongoing Fukushima crisis meant Japan must turn to other energy sources.

"The large-scale, long-running nuclear accident has triggered radiation leakage, causing serious concerns not only in Japan but also in the world," Kan, in a black suit and tie, said at a memorial ceremony in Hiroshima's Peace Park.

"I will reduce Japan's reliance on nuclear power, aiming at creating a society that will not rely on atomic power generation," he added.

The March 11 earthquake and tsunami triggered the ongoing nuclear accident, which has leaked radiation into air, soil and sea and forced tens of thousands to leave their homes, leading to rising public anger.

In more rare remarks on energy policy, Hiroshima mayor Kazumi Matsui also called for the government to review its sources of power after Fukushima, the world's worst nuclear accident since Chernobyl 25 years ago.

"The continuing radiation scare has made many people live in fear and undermined people's confidence in nuclear power," he said.

"The Japanese government must quickly review the energy policy... to regain people's understanding and trust," he said.

A city official said around 50,000 people took part in the ceremony to remember the 1945 atomic bombing, which killed an estimated 140,000 people instantly or due to burns and radiation sickness soon after the blast.

Over 70,000 perished as a result of another US atomic attack on the port of Nagasaki three days later.

Saturday's ceremony was attended by representatives of more than 60 countries including the United States.

Kan, who also plans to attend a ceremony to mark the Nagasaki bombing on Tuesday, stressed in his speech: "We must never forget the calamity of a nuclear arm that attacked Hiroshima 66 years ago. We must never let it happen again."

At a press conference in Hiroshima afterwards, Kan said a nuclear accident and an atomic bombing share something in common -- causing worries due to spreading radiation.

"I felt even more strongly (after the accident) about the importance to seek a society free from nuclear weapons, a society that doesn't trigger problems due to radiation," he said.

"The government's policy and my speech (at the ceremony) correspond with each other."

Kan, a one-time environmental activist, has pledged to boost alternative energy sources to 20 percent of the nation's energy mix by the 2020s. They currently make up about nine percent, most of it hydroelectric power.

But the embattled prime minister has been fighting calls to resign amid rock-bottom poll ratings, while his government is still struggling to control the crippled Fukushima nuclear plant.

The government and operator Tokyo Electric Power Co. (TEPCO) are aiming to bring the damaged nuclear reactors at the plant to a state of cold shutdown by January.