Some 50,000 people stood for a minute of silence in Hiroshima's peace park near the epicenter of the early morning blast on August 6, 1945, that killed up to 140,000 people.
The bombing of Nagasaki three days later killed tens of thousands more, prompting Japan's surrender to the World War II Allies.
Prime Minister Shinzo Abe, among many dignitaries attending the event, said that as the sole country to face nuclear attack, Japan has the duty to seek to wipe out nuclear weapons, AP reported.
He made no mention of the dilemma this resource-scarce country is facing over nuclear energy, nor of the tens of thousands of people displaced by risks from radioactivity from a nuclear disaster in Fukushima, in its northeast.
Most of Japan's nuclear power plants were taken offline after the massive earthquake and tsunami in 2011 damaged reactors at a plant in Fukushima, causing meltdowns.
Abe favors restarting plants under new safety guidelines, while many Japanese oppose such restarts.