Hiroshima marks atomic anniversary

Tens of thousands have gathered at a peace memorial park in Hiroshima to mark the 68th anniversary of the US atomic bombing of the city, as anti-atomic sentiment runs high in Japan.

The annual ceremony came as radioactive water leaks at the crippled Fukushima nuclear plant have stoked renewed fears about the plant's precarious state, and underscored broader worries about atomic power following Japan's 2011 nuclear crisis.

In Hiroshima, ageing survivors, relatives, government officials and foreign delegates observed a moment of silence at 8:15am, the time of the detonation which turned the western Japanese city into a nuclear inferno.

"We offer heartfelt consolation to the souls of the atomic bomb victims by pledging to do everything in our power to eliminate the absolute evil of nuclear weapons and achieve a peaceful world," Hiroshima Mayor Kazumi Matsui told the ceremony.

An American B-29 bomber named Enola Gay dropped an atomic bomb on Hiroshima August 6, 1945, in one of the final chapters of World War II. It killed an estimated 140,000 by December that year. Three days later, the port city of Nagasaki was also bombed, killing an estimated 70,000 people.

The Allied powers have long argued that the twin attacks brought a quick end to the war by speeding up Japan's surrender, preventing millions more casualties from a planned land invasion.

Anti-nuclear sentiment has flared in Japan after an earthquake-sparked tsunami left some 19,000 dead or missing and knocked out cooling systems at the Fukushima Daiichi nuclear plant two years ago.

Meltdowns at the crippled site spread radiation over a large area and forced thousands to leave their homes in the worst atomic disaster in a generation.

Concerns about Fukushima remain high since the accident. In recent weeks, the plant's operator Tokyo Electric Power admitted for the first time that radioactive water had leaked into the ocean as it struggles to contain the toxic build-up.