Some 50 000 people, including atomic bomb survivors as well as Prime Minister Taro Aso and representatives from more than 50 nations, gathered at a memorial to the dead.
Hiroshima Mayor Tadatoshi Akiba praised US President Barack Obama for his anti-nuclear views as he delivered a speech at the memorial, within sight of the A-bomb dome, a former exhibition hall burned to a skeleton by the bomb's intense heat.
The mayor noted Obama said in an address that as the only nuclear power to have used an atomic weapon, the United States has "a moral responsibility to act" to realise a nuclear-free world.
"Nuclear weapons abolition is the will not only of the hibakusha (atomic bomb survivors) but also of the vast majority of people and nations on this planet," he said.
"We refer to ourselves, the great global majority, as the 'Obamajority', and we call on the rest of the world to join forces with us to eliminate all nuclear weapons by 2020."
Those attending the memorial ceremony stood up and offered silent prayers at 08:15, the exact moment in 1945 when the bomb was dropped.
The single bomb killed some 140 000 people in 1945 alone. They died instantly or in the days and weeks that followed as radiation or horrific burns took their toll.
Three days after the Hiroshima bombing, the United States dropped a second nuclear bomb on Nagasaki, which killed another 70 000 people in the southern port city.
Japan surrendered in World War II on August 15. The nation has since been officially pacifist and become one of the United States' closest allies, hosting some 47 000 US troops.