A total of 74 nations were represented at this year's memorial event.
'Voices of peace'
Al Jazeera's Steve Chao, reporting from Hiroshima, said many Japanese felt the US presence at the ceremony was an affirmation of Obama's seriousness in working towards a nuclear-free world, but warned that it would be an immense task to achieve.
"People here are under no illusion of the challenges ahead, the fact that it might be difficult for the US congress to approve the stepdown in terms of the nuclear stockpile and other challenges posed by nations like North Korea, and also the fact that there remains 25,000 nuclear warheads in the hands of various nations around the world.
Al Jazeera's Steve Chao reports on the message of hope at the Hiroshima memorial
"But there is a renewed sense of hope in Hiroshima and many people here have dedicated their entire lives working towards peace."
Tadatoshi Akiba, the mayor of Hiroshima, praised efforts to achieve nuclear disarmament, saying he was "encouraged that our voice is being heard".
"It is the wish of the survivors of the bombing that the voices of peace will be heard," he said in a speech at the ceremony.
Akiba called on the Japanese government to take a leadership role in disarmament and "turning a new page in human history".
About 1,000 white doves, symbolising peace, were released as Naoto Kan, the Japanese prime minister, addressed the ceremony.
"The human race must not repeat the horror and misery caused by atomic bombs," he said.
"Japan, as the only nation to have been attacked by the war-time atomic bombs, has a moral responsibility to lead the efforts toward realisation of a world without nuclear weapons."
The US and Russia still have more than 22,000 nuclear warheads between them, while France, Britain, China, India, Pakistan and Israel have a combined total of about 1,000 weapons, according to the International Commission on Nuclear Non-proliferation and Disarmament.
The global nuclear weapons stockpile is equivalent to about 150,000 Hiroshima bombs.
"Little Boy", the four-tonne uranium bomb detonated over Hiroshima in 1945, caused a blinding flash and a fireball hot enough to melt sand into glass and vaporise every human within a 1.6-kilometre radius.
An estimated 140,000 people died instantly in Hiroshima or succumbed to burns and radiation sickness soon after the blast, and more than 70,000 perished as a result of another US atomic attack on the port of Nagasaki three days later.
|About 140,000 people died in the days after the explosion 65 years ago [Reuters]
"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 Ki-moon, the UN secretary general, said.
"For as long as nuclear weapons exist, we will live under a nuclear shadow."
At Hiroshima's Peace Memorial Park, where Friday's ceremony was held, some visitors expressed concern that Japan's view of the bombing – seen by many as excessive use of deadly force - conflicts with the US view.
Katsuko Nishibe, a 61-year-old peace activist, said she welcomed the US decision to send Roos, but added that it should not lead to an acceptance that it was the right thing to do.
"I don't think it was necessary," she said. "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."