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 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 U.S. 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 on 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 accident at the Fukushima nuclear power plant struck by a tsunami 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 on the experience to future generations so the lessons of Hiroshima are not forgotten.
Daniel is the oldest grandchild of Harry and Bess Truman. He has published two books, a memoir and a collection of his grandparents' letters.