TOKYO — Japan marked the 67th anniversary of the world's first atomic bomb attack with a ceremony Monday that was attended by a grandson of Harry Truman, the US president who ordered the bomb dropped on the city of Hiroshima. About 50,000 people gathered in Hiroshima's peace park near the epicenter of the 1945 blast that destroyed most of the city and killed as many as 140,000 people. A second atomic bombing Aug. 9 that year in Nagasaki killed tens of thousands more and prompted Japan to surrender to the World War II allies. The ceremony, attended by representatives of about 70 countries, began with the ringing of a temple bell and a moment of silence. Flowers were placed before Hiroshima's eternal flame, which is the park's centerpiece. Truman's grandson, Clifton Truman Daniel, and the grandson of a radar operator who was on both of the planes that dropped the atomic bombs, joined in the memorial. Ari Beser's grandfather, Jacob Beser, was the only person who directly took part in both the Hiroshima and Nagasaki bombings. In a news conference after the memorial, Daniel declined to comment on whether his grandfather's decision was the right one. “I'm two generations down the line. It's now my responsibility to do all I can to make sure we never use nuclear weapons again," he said, according to Japan's Kyodo news service. Daniel, 55, said in a statement earlier that he decided to visit Hiroshima and Nagasaki because he needed to know the consequences of his grandfather's decision as part of his own efforts to help achieve a nuclear-free world. The US government sent a representative — the American ambassador — to the annual commemoration for the first time two years ago. Ambassador John Roos attended the Hiroshima ceremony Monday. Hiroshima Mayor Kazumi Matsui said Japan must take a bolder role in leading global disarmament efforts and called on world leaders to come to his city to “contemplate peace." He also said the nuclear accident at Fukushima last year has shown the dangers of nuclear technology, even for peaceful purposes, and urged the government to create a mix of energy sources for Japan that is safe and secure. “I firmly believe that the demand for freedom from nuclear weapons will soon spread out from Hiroshima, encircle the globe, and lead us to genuine world peace," he said. Matsui noted that the average survivor of the bombing is now 78 years old, and said the city is increasing its effort to provide them with health care and chronicle their experiences so the events of that day are remembered. Prime Minister Yoshihiko Noda said Japan must pass the experience on to future generations so that the lessons of Hiroshima are not forgotten. Usually sedate Japan has seen a string of anti-nuclear protests since Prime Minister Noda in June ordered the restart of two reactors. Noda defended the move citing looming power shortages after Japan switched off its 50 nuclear reactors — which once provided the resource-poor country with a third of its energy — in the wake of the Fukushima disaster. Weekly demonstrations outside the prime minister's official residence have drawn thousands while a rally in west Tokyo last month saw a crowd that organizers claimed swelled to 170,000. The commemoration ceremony comes nearly a year and a half after a quake-sparked tsunami, which left some 19,000 dead or missing, knocked out cooling systems at the Fukushima plant, causing meltdowns that spread radiation over a large area and forced thousands to evacuate.Some have moved back to nearby communities, but many more have not and there are fears it could be decades before the area was deemed safe to live in. — Agencies