Japan observed a minute of silence Tuesday to mark the 68th anniversary of the nuclear bombing of Hiroshima.
Survivors and relatives of victims were among 50,000 people gathered at a peace park in Hiroshima for a somber ceremony.
Prime Minister Shinzo Abe told the crowd Japan has a unique responsibility to push for the end of nuclear weapons.
"We Japanese are history's sole victims of the nuclear attack of the nuclear attack and we have the certain responsibility to bring about a world without nuclear weapons," Abe said. "And it is our duty to continue to remind the world of [nuclear weapons'] inhumanity."
Hiroshima Mayor Kazumi Matsui made a similar pledge, calling nuclear bombs the "ultimate inhumane weapon and an absolute evil."
About 140,000 were killed in the days following the August 6, 1945 U.S. bombing of Hiroshima. Three days later, U.S. planes dropped a nuclear bomb on Nagasaki, killing about 70,000 more.
The U.S. and its allies have argued the bombings were necessary and helped save lives by convincing Japan to surrender, bringing a quicker end to World War II.
The sensitive anniversary comes as Japan wrestles over a debate about the role of nuclear energy, following the country's 2011 earthquake and nuclear disaster.
Almost all of Japan's nuclear power plants remain shut down following the meltdowns at Fukushima, which spread radiation over a large area and forced thousands to flee.
Prime Minister Abe and his party want to restart the plants following safety inspections, but the plan has proved controversial for many in the energy-dependent nation.
Since the accident, there have been repeated safety concerns at the Fukushima power plant, where operators are struggling to contain radiation-contaminated water.