- Created on Monday, 06 August 2012 11:06
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Tens of thousands of people attended the event, amid growing anti-nuclear sentiment and protests in the country.
A bell marked the start of a one-minute silence at 08:15 local time (23:15 GMT) when the US bomber Enola Gay dropped the bomb that killed 140,000 people.
Prime Minister Yoshihiko Noda, at the ceremony, said the lessons of Hiroshima must not be forgotten.
"We will establish an energy mix with which people can feel safe in the long- and medium-term, based on our policy that we will not rely on nuclear power," he said.
Mr. Noda has been under pressure from anti-nuclear activists since he ordered the restart of two nuclear reactors in June.
All 50 of Japan's nuclear plants were shut after the meltdown at Fukushima, which was triggered by a tsunami and earthquake in March 2011. The crisis was regarded as the worst nuclear disaster since Chernobyl in 1986.
The ceremony was also attended by a grandson of former US President Harry Truman, who ordered the bombing of Hiroshima and then Nagasaki.