Japanese Prime Minister Naoto Kan has repeated a pledge to reduce reliance on nuclear power, as people mark the 66th anniversary of the Hiroshima bombing.
Thousands gathered at the city's peace memorial to observe one minute's silence in memory of the 140,000 killed by the US atomic attack in 1945.
Mr Kan used the occasion to address the crisis caused by a tsunami wrecking the Fukushima Daiichi power plant in March.
He promised to challenge "conventional beliefs" that nuclear energy was safe.
The Fukushima plant continues to leak radioactive material, nearly five months after a 9.0 magnitude earthquake triggered the tsunami which caused the damage.
It was causing serious concerns not just in Japan but across the world, Mr Kan said.
"We will deeply reflect over the conventional belief that nuclear energy is safe, thoroughly look into the cause of the accident and - to secure safety - implement fundamental measures," he said.
About 30% of Japan's electricity was nuclear generated before the Fukushima crisis, and the country had previously targeted raising that figure to 53% by 2030.
But Mr Kan said: "I will reduce Japan's reliance on nuclear power, aiming at creating a society that will not rely on atomic power generation."
The prime minister spoke after laying a wreath of yellow flowers at Hiroshima Peace Memorial Park, where doves were released as a symbol of peace.
Japan has long vowed never to make or possess nuclear weapons but had embraced nuclear power as it rebuilt after World War II.
However, referring to Fukushima, Hiroshima mayor Kazumi Matsui told those gathered: "The continuing radiation scare has made many people live in fear and undermined people's confidence in nuclear power.
"The Japanese government must quickly review the energy policy... to regain people's understanding and trust," added Mr Matsui, the son of an atomic bomb survivor.