Around 140,000 people died in Hiroshima on August 6, 1945 and 74,000 in Nagasaki three days later, when the United States dropped atomic bombs on the two Japanese cities days before the end of World War II.
"Japan, as the only nation to have suffered atomic bombings in war, will continue efforts towards a nuclear-free world," Prime Minister Fumio Kishida said at a ceremony in Hiroshima.
"The path towards it is becoming increasingly difficult because of deepening divisions in the international community over nuclear disarmament and Russia's nuclear threat," he said.
"Given this situation, it is all the more important to bring back international momentum towards realisation of a nuclear-free world," he said.
"Devastation brought to Hiroshima and Nagasaki by nuclear weapons can never be repeated," said Kishida, whose family comes from Hiroshima.
Kishida's comments echoed those of UN chief Antonio Guterres, who issued a statement on the Hiroshima anniversary saying that "some countries are recklessly rattling the nuclear sabre once again, threatening to use these tools of annihilation."
"In the face of these threats, the global community must speak as one. Any use of nuclear weapons is unacceptable," Guterres said.
At the ceremony, thousands of people -- survivors, relatives and foreign dignitaries from a record 111 countries -- prayed for those killed or wounded in the bombing and called for world peace.
Russia and Belarus were not invited to the ceremony for the second straight year because of the Ukraine crisis.
Participants, many dressed in black, offered a silent prayer at 8:15 am (2315 GMT Saturday) when the first nuclear weapon used in wartime was dropped.
- Nuclear disarmament -
Kishida hosted the G7 summit in the city earlier this year.
Kishida has tried to move nuclear disarmament up the global agenda, taking leaders of wealthy democracies to Hiroshima's peace park memorials and museum.
However, there is little appetite to reduce stockpiles with Russia repeatedly issuing thinly veiled warnings that Moscow could use nuclear weapons in Ukraine, as well as repeated North Korean missile tests and stalling efforts towards non-proliferation.
Earlier this month, more than 100 medical journals across the world issued a rare joint call for urgent action to eliminate nuclear weapons, warning that the threat of nuclear catastrophe was "great and growing."
The anniversary follows the US release of blockbuster "Oppenheimer," a biopic about J. Robert Oppenheimer, one of the creators of atomic weapons.
No release date has been announced for Japan and there is speculation it may not show in cinemas at all.
"Seventy-eight years went by and people are starting to forget, so it's a good moment to make a movie and remind us about what happened," Ryo Kento, a student, told AFP in Tokyo ahead of the Hiroshima anniversary.
Last week, Japanese social media users expressed outrage after Internet memes referencing the films "Barbie" and "Oppenheimer" were circulated using the hashtag #Barbenheimer.
Memes shared on social media combined images from both, with one showing a cheering Barbie -- a wildly popular children's doll -- on the shoulders of Oppenheimer, against the backdrop of an apocalyptic blast.
Warner Bros. Japan, the local distributor of "Barbie," later apologised for appearing to back the memes' circulation.
"As many died, I don't think it's something to be made fun of," Kento said.
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