Japan is marking the 70th anniversary of the United States dropping an atomic bomb on Hiroshima, which led to the end of World War II.
Tens of thousands of people observed a minute of silence Thursday as bells tolled at the Peace Memorial Park, near the epicenter of the bombing.
Hiroshima Mayor Kazumi Matsui called for a renewed effort to rid the world of nuclear weapons, which he called "pure evil."
"In order for us to live together, we need to end the use of all nuclear weapons, the ultimate in inhumane, pure evil," Matsui said. "And the moment to get this done is now."
Prime Minister Shinzo Abe said Japan, as the only country ever to be attacked by an atomic bomb, has a unique responsibility to create a world without nuclear weapons.
"And that resolve translates in us proposing a new draft resolution at the United Nations in the fall on nuclear disarmament," Prime Minister Abe pledged.
The Hiroshima bombing killed around 140,000 people, either instantly or from radiation burns in the immediate aftermath. Another bomb was dropped on the port city of Nagasaki three days later, killing an estimated 70,000 residents.
Washington argued the attacks were necessary to bring about a quicker end to World War II. Six days after the Nagasaki attack, Japan surrendered, ending the war.
U.S. Secretary of State John Kerry on Thursday said the anniversary was a powerful reminder of the impact of war and of global efforts to reduce the number of nuclear weapons.
"It also underscores the importance of the agreement we've reached with Iran to reduce the possibility of more nuclear weapons," said Kerry, who is attending a meeting of regional leaders in Kuala Lumpur.
The ceremony in Hiroshima was attended by U.S. Ambassador to Japan Caroline Kennedy, U.S. Under Secretary for Arms Control and International Security Rose Gottemoeller, as well as representatives from more than 100 other countries.
The anniversary comes as Japan is engaged in a public debate over the use of nuclear energy, as well as efforts to expand the role of the country's military.
The hawkish Prime Minister Abe is attempting to restart the country's nuclear reactors that were shut down following the Fukushima earthquake and tsunami disaster in 2011. He is also trying to force through a series of security bills that could open the possibility of Japanese troops fighting in combat for the first time since World War II.
Hiroshima mayor Matsui used Thursday's ceremony to criticize Abe's security plans, saying leaders should stick with the "pacifism of the Japanese constitution."