Hiroshima: 'There was a flash. Then everything turned black'

It was a day for remembering the victims of one of history's greatest mass killings, but most of the attention was on a middle-aged man sitting among the 55,000 people watching yesterday's memorial ceremony in Hiroshima.

John Roos, the US ambassador to Japan, was one of 74 foreign envoys who fell silent at 8.15am – the time "Little Boy" detonated over the city on 6 August 1945. But for survivors of the blast that reduced Hiroshima to ashes and took 140,000 lives, his presence loomed by far the largest.

"It's very important to us that he came," said Hiroshi Takayama, 80, who was working in a factory with his schoolmates when he felt the searing blast from the bomb. "We've been telling people for 65 years not to use these weapons, but these people have never showed up until now. If they cared, they would have come to see the facts for themselves."

The ceremony was Hiroshima's largest ever, with choirs of children joining the elderly survivors, descendants and dignitaries. It began with the symbolic offering of water: many who died after the blast complained of an excruciating thirst in their final moments. Yesterday, under a burning sun, prayers were offered to the dead and bells rang out.

Known in Japan as hibakusha, the survivors believe that the decision by the US, Britain and France to send official delegates to the city for the first time is a sign that the global momentum for disarmament is building. But Japan's Prime Minister, Naoto Kan, appeared to dash their hopes when he defended the US nuclear deterrent, saying it was "necessary for our nation".

"Of course that's what we want," said Yoshimichi Ishimaru, who is the son of a Hiroshima hibakusha. "We know that if we scrap the nuclear deterrent, we have to begin to question the whole defence treaty with America, and that's very difficult. But Hiroshima can't just think about the national interest – it must consider the whole of humanity."

Hiroshima people have long resented the political calculations needed to maintain Japan's 60-year-old military alliance with its old enemy. Many say the nation's decision never to produce, possess or allow the entry of nuclear weapons into the country has been betrayed by US and Japanese leaders.

Washington and Tokyo struck a backroom deal decades ago allowing nuclear-armed US ships and aircraft to pass through or over Japanese territory. Former prime minister Eisaku Sato, who helped broker the deal, won the 1974 Nobel Peace Prize for his opposition to any plans for a Japanese nuclear-weapons program.

Survivors are pinning their hopes on Barack Obama, who they believe will be the first American leader to visit Hiroshima. "We hope that Mr Obama will come and see with his own eyes what happens after you use a nuclear weapon," said Akihiro Takahashi, 79, a former director of the Hiroshima Peace Memorial Museum. "If he comes, we would be one step closer to total abolition."

Mr Takayama recalls hearing the drone of a single B29 bomber flying overhead as he worked in the factory on that fateful morning. "All of a sudden there was an extremely bright flash and everything in the world was bathed in intense light and heat. Then everything turned pitch black." The presence of Mr Roos yesterday means that "a shut door has been opened", he said. Eventually, he says, Mr Obama may come too. "From him, I want all to know the importance of stopping nuclear weapons."