Tens of thousands of people gathered for peace ceremonies in Hiroshima on Wednesday, marking the 69th anniversary of the US atomic bombing of the city, as anti-nuclear sentiment runs high in Japan.

Bells tolled as ageing survivors, relatives, government officials and foreign delegates observed a moment of silence in the rain at 8:15 am local time (2315 GMT), when the detonation turned the western Japanese city into an inferno.

People attending Wednesday's ceremony placed flowers in front of the cenotaph at Peace Memorial Park in downtown Hiroshima.

The city's mayor Kazumi Matsui recalled the grim memories of one survivor at a ceremony attended by Prime Minister Shinzo Abe and US ambassador to Japan Caroline Kennedy.

The survivor, a 15-year-old pupil at the time, remembered hearing "voices from the brink of death" begging for "water, please".

"The pleas were from younger students," the mayor said, recounting the survivor's grisly description of "their badly burned, grotesquely swollen faces, eyebrows and eyelashes singed off, school uniforms in ragged tatters".

Many survivors -- known in Japan as "hibakusha" -- feel profound guilt over living through the attack, Matsui said.

But "people who rarely talked about the past because of their ghastly experiences are now, in old age, starting to open up", he added.

Shigeji Yonekura, a 81-year-old Hiroshima survivor, told AFP: "It's sad to see my fellow hibakusha die year after year, but I want to keep telling young people about my horrific experience for as long as I live."

An American B-29 bomber named Enola Gay dropped an atomic bomb on Hiroshima on August 6, 1945, in one of the final chapters of World War II. It had killed an estimated 140,000 by December that year.

On August 9, the port city of Nagasaki was also bombed, killing an estimated 70,000 people.

Japan surrendered days later -- on August 15, 1945 -- bringing the war to a close.

'Sombre reflection'

Opinion remains divided over whether the twin attacks were justified. While some historians say that it prevented many more casualties in a planned land invasion, critics have said the attacks were not necessary to end the war, arguing that Japan was anyway heading for imminent defeat.

Paul Tibbets, who piloted the Enola Gay, said he never had any second thoughts about dropping the bomb, telling a newspaper in an interview in 2002, five years before his death, "I knew we did the right thing".

The last surviving crewman of the Enola Gay, Theodore Van Kirk, died only last week, at the age of 93. His funeral was reportedly scheduled for August 5 in Pennsylvania, just hours before the Hiroshima commemorations in Japan.

Washington, which has been a close ally of Tokyo since the war, has never officially apologised for the bombings, however, leaked diplomatic cables from 2009 suggested that the Japanese government had rebuffed the idea of a US apology and a visit to Hiroshima by President Barack Obama.

But US diplomats have attended the annual commemorations of the attacks. And two years ago, a grandson of former US President Harry Truman, who gave the order to drop the bombs, attended peace ceremonies in Hiroshima.

In a statement issued on Wednesday's anniversary, Kennedy, the US ambassador said: "This is a day for sombre reflection and a renewed commitment to building a more peaceful world."

Anti-nuclear sentiment flared in Japan after an earthquake-sparked tsunami left some 19,000 dead or missing and knocked out cooling systems at the Fukushima nuclear plant on the northeast coast in 2011.

None of those deaths were directly attributed to the nuclear crisis but the reactor meltdowns spread radiation over a large area and forced thousands to leave their homes in the worst atomic disaster since Chernobyl in 1986.

Despite strong public opposition, Japan's nuclear watchdog last month said that two atomic reactors were safe enough to switch back on.

The decision marked a big step towards restarting the country's nuclear plants, which were shut after the disaster.