The highlight of the event was the traditional Peace Declaration delivered at the ceremony in the Peace Memorial Park by Hiroshima Mayor Kazumi Matsui, Tokyo-based Kyodo News reported.
Hailing the Group of Seven leaders' landmark visit in May to the Japanese park and its atomic bomb museum, Matsui urged policymakers to abandon the idea that nuclear weapons deter war.
The gruesome nuclear legacy took center stage at the last G-7 summit of major economies in May in Japan.
"Leaders around the world must confront the reality that nuclear threats now being voiced by certain policymakers reveal the folly of nuclear deterrence theory," Matsui said, adding: "They must immediately take concrete steps to lead us from the dangerous present toward our ideal world."
A moment of silence was observed at 8.15 a.m., the exact time when the uranium bomb was dropped by the US bomber Enola Gay and detonated over the city on Aug. 6, 1945, killing an estimated 140,000 people by the end of the year.
Japanese Prime Minister Fumio Kishida, who attended the event, said: "The path to nuclear disarmament has become more perilous due to deepening international divisions and nuclear threats by Russia.
"It is crucial to reinvigorate international momentum toward a 'world without nuclear weapons' once again."
It has been 78 years since the US dropped atomic bombs on Hiroshima on Aug. 6 and Nagasaki on Aug. 9 in 1945 during World War II in the Pacific.
The bombs contained enriched uranium and had a blast yield of 13 kilotons of TNT.
The Hiroshima bombing destroyed everything within 1.5 kilometers (1 mile) of ground zero and created searing heat of 3,000 degrees Celsius (5,432 degrees Fahrenheit) at its center. The bomb destroyed 70% of Hiroshima.
While many died without getting medical support, those who went to the city for help died from radioactive rain.
It is known that the reason then-US President Harry Truman ordered the use of atomic bombs was to offset the Soviet threat in East Asia and Eastern Europe, as well as to make a show of strength to the USSR.
While the Americans announced that the death toll from the atomic bombings was 117,000, the Japanese said it was close to half a million. Survivors, called "Hibakusha," suffer from cancer, disfigurement, and hard-to-treat diseases.