About 50,000 people, including representatives from 80 nations, gathered for an annual ceremony at Hiroshima Peace Memorial Park Sunday, reports The Japan Times.
Prime Minister Shinzo Abe called for global cooperation to end nuclear weapons.
"For us to truly realize a world without nuclear weapons, the participation of both nuclear weapon states and non-nuclear weapon states is necessary," he said.
Last month, the United Nations reached its first agreement to ban nuclear weapons. But Japan, along with the nine nuclear-armed nations, including the United States, refused to take part in the negotiations and the vote.
Critics of the treaty, including the United States, say it does nothing to counter the "grave threat posed by North Korea's nuclear program." Member states can sign the Treaty on the Prohibition of Nuclear Weapons next month; if ratified, it will be put into effect 90 days later, reports The New York Times.
Japan already adheres to a policy of not possessing, producing or allowing nuclear weapons on its territory. It is the only country to have ever come under nuclear attack.
72 years ago
On Aug. 6, 1945, the atomic bomb dropped by the Enola Gay Boeing B-29 detonated, killing an estimated 140,000 people. Three days later, the U.S. dropped a second bomb on the city of Nagasaki, killing an estimated 75,000. Within weeks, Japan surrendered.
Last year, then-President Obama became the first U.S. president to visit Hiroshima, where he called for an end to nuclear weapons. That visit provoked the ire of then-candidate Trump.
"Does President Obama ever discuss the sneak attack on Pearl Harbor while he's in Japan? Thousands of American lives lost," Trump tweeted.
So far, the Trump administration has focused on a denuclearized North Korea over a denuclearized world.
In February Trump told Reuters that "if countries are going to have nukes, we're going to be at the top of the pack."
Sanctions appear to be the current international tool of choice for taking on North Korea.
A day before the Hiroshima anniversary, the United Nations Security Council unanimously voted to impose new sanctions on North Korea over last month's long-range missile tests.
On Sunday, U.N. Secretary General António Guterres issued a message calling for the United States and other nuclear-armed countries to do more to rid the world of nuclear weapons.
"(O)ur dream of a world free of nuclear weapons remains far from reality. The states possessing nuclear weapons have a special responsibility to undertake concrete and irreversible steps in nuclear disarmament."