Despite some expectations of progress toward nuclear disarmament in recent years, Mayor Kazumi Matsui warned of the re-emergence of tensions over nuclear weapons seen during the Cold War and sought rational actions by global leaders.
"If the human family forgets history or stops confronting it, we could again commit a terrible error," Matsui said at a memorial ceremony to remember the atomic bombing, Kyodo news reported on Monday.
Japan needs to lead the international community toward "dialogue and cooperation for a world without nuclear weapons," the mayor added.
At the Peace Memorial Park, a moment of silence was observed at 8:15 a.m., when the "Little Boy" uranium-core atomic bomb dropped by a US bomber exploded above Hiroshima on Aug. 6, 1945. It killed an estimated 140,000 people by the end of that year.
Matsui expressed hope that the easing of tensions on the Korean Peninsula will continue through dialogue and called on global leaders to make an international treaty comprehensively prohibiting nuclear weapons a "milestone" toward the goal of ridding the world of nuclear arsenals.
The mayor made the Peace Declaration before representatives from 85 countries and the European Union. The United States sent its ambassador to Japan to the annual ceremony for the first time in three years.
"Certain countries are blatantly proclaiming self-centered nationalism and modernizing their nuclear arsenals, rekindling tensions that had eased with the end of the Cold War," Matsui said.
Prime Minister Shinzo Abe, attending the memorial ceremony, pledged that Japan will try to bridge nuclear powers and non-nuclear states and lead international efforts.
"Maintaining its three non-nuclear principles, our country is determined to make strenuous efforts to serve as a bridge between both parties," Abe said.
As of March, the number of hibakusha stood at 154,859. Their average age is now just over 82.
UN Secretary-General Antonio Guterres said in his message the legacy of Hiroshima is one of "resilience" and sought continued moral support from hibakusha survivors.