A U.S. representative participated for the first time Friday in Japan's annual commemoration of the American atomic bombing of Hiroshima, in a 65th anniversary event that organizers hope will bolster global efforts toward nuclear disarmament.

The site of the world's first A-bomb attack echoed with the choirs of schoolchildren and the solemn ringing of bells Friday as Hiroshima marked its biggest memorial yet. At 8:15 a.m. -- the time the bomb dropped, incinerating most of the city -- a moment of silence was observed.

Hiroshima's mayor welcomed Washington's decision to send U.S. Ambassador John Roos to Friday's commemoration, which began with an offering of water to the 140,000 who died in the first of two nuclear bombings that prompted Japan's surrender in World War II.

Mayor Tadatoshi Akiba is also hoping that President Barack Obama will visit Hiroshima, an idea that Obama has said he would like to consider but that would be highly controversial and unprecedented for a sitting U.S. president.

"We need to communicate to every corner of the globe the intense yearning of the survivors for the abolition of nuclear weapons," Akiba told the 55,000 people at the ceremony.

Akiba called on the Japanese government to take a leadership role in nuclear disarmament toward "turning a new page in human history."

"I offer my prayers to those who died -- we will not make you be patient much longer."

Along with the U.S., Britain and France also made their first official appearance at the memorial, as well as U.N. Secretary-General Ban Ki-moon. Altogether, 74 nations were represented.

China, which sent a low-ranking official in 2008, was not participating. Officials said Beijing did not give a reason.

Hiroshima was careful to ensure that the memorial -- while honoring the dead -- emphasized a forward-looking approach, focusing not on whether the bombing was justified, a point which many Japanese dispute, but on averting any future nuclear attacks.

Roos said the memorial was a chance to show resolve toward nuclear disarmament, which Obama has emphasized as one of his administration's top objectives.

"For the sake of future generations, we must continue to work together to realize a world without nuclear weapons," Roos said in a statement.

Ban, who presented flowers at the Eternal Flame in Hiroshima's Peace Memorial Park, said this year's memorial will send a signal to the world that nuclear weapons must be destroyed.

"Life is short, but memory is long," Ban said. "For many of you, that day endures ... as vivid as the white light that seared the sky, as dark as the black rains that followed."

Ban added that the time has come to move from "Ground Zero, to Global Zero" -- a world without any nuclear arms.

Washington's decision to attend the anniversary has been welcomed by Japan's government, but has generated complex feelings among some Japanese who see the bombing as unjustified and want the United States to apologize.

"I'm not sure if I would welcome President Obama here," said Katsuki Fujii, a 20-year-old college student. "I don't think we have the same idea what peace is. He seems to think some wars are good and some are bad -- I think they are all bad."

About 140,000 people were killed or died within months when the American B-29 "Enola Gay" bombed Hiroshima on Aug. 6, 1945. Three days later, about 80,000 people died after the United States also bombed Nagasaki.

Japan surrendered on Aug. 15, ending World War II.

The United States decided to drop the bombs because Washington believed it would hasten the end of the war and avert the need to wage prolonged and bloody land battles on Japan's main island. That concern was heightened by Japan's desperate efforts to control outlying islands such as Iwo Jima and Okinawa as the Allies closed in.

Concerns that attending the anniversary ceremony would reopen old wounds had kept the U.S. away until this year.

Former President Jimmy Carter visited Hiroshima's Peace Museum in 1984, years after he was out of office. The highest-ranking American to visit while in office is House Speaker Nancy Pelosi, who went in 2008. Roos also visited Hiroshima soon after assuming his post last year.

U.S. Secretary of State Hillary Clinton told reporters in Washington on Thursday that Obama believed "it would be appropriate to recognize this anniversary" by sending Roos to the annual memorial.

The State Department deemed the time was right to do so, and it was a chance to push Obama's own goal of nuclear disarmament.

At Hiroshima's Peace Memorial Park, where Friday's ceremony was held, leftist groups in trucks blared anti-U.S. slogans to the crowds.

"The bombing of Hiroshima was totally unnecessary," said one group. "U.S., take your nukes and go home."

Still, Obama remains a popular figure in Japan -- Obama T-shirts are on sale at the Peace Park's museum -- and many would welcome a visit.

Katsuko Nishibe, a 61-year-old peace activist, said she welcomed the decision to send Roos, but added that she thought it was dangerous to think that the bombing of Hiroshima was justified.

"We have a very different interpretation of history. But we can disagree about history and still agree that peace is what is important. That is the real lesson of Hiroshima."