"If the human family forgets history or stops confronting it, we could again commit a terrible error," Mayor Kazumi Matsui said at the ceremony.
Survivors of the bombing known as hibakusha were also in attendance at the annual ceremony.
Some of the hibakusha, many now aged over 82, have been working with the International Campaign to Abolish Nuclear Weapons to help a treaty to be adopted by the United Nations to ban nuclear weapons.
However, Japan has not become a signatory to the Treaty on the Prohibition of Nuclear Weapons which came into effect in July 2017.
UN Secretary General Antonio Guterres said in his message during the ceremony that Hiroshima's legacy is one of "resilience" and sought continued moral support from the hibakusha survivors for efforts in promoting the ban of nuclear weapons.
But as with years past and, perhaps, for those in the future, while Japan has a tendency to focus solely on the inward tragedy that nuclear and chemical warfare has inflicted on it, many experts on the matter hope that Japan will also take the time to remember that its own involvements in World War II had also brought immeasurable suffering.
For example, the Imperial Japanese Army's notorious Unit 731, which was based in the Pingfang district of Harbin, the largest city then in northeast China, was set up around 1936 and conducted vivisection experiments on live human beings to test germ-releasing bombs and chemical bombs, among other criminal atrocities.
The unit became Japan's top-secret biological and chemical warfare research base and operated as the nerve center of Japanese biological warfare in China and Southeast Asia during World War II.
At least 3,000 people were used for human experimentation by Unit 731 along with a small percentage of Soviets, Mongolians, Koreans, and soldiers of the Allied Forces who had been taken captive. Some of those killed in ways unimaginable were just children.
More than 300,000 people across China were killed by Japan's biological weapons during WWII.
The notorious Unit 731 managed to keep its atrocities largely concealed due to the International Military Tribunal for the Far East not prosecuting the unit's commanders under condition they handed over the germ warfare data to the United States.
Right wing forces here have also, since the unit's abominable crimes committed before and during WWII, attempted to sequester the facts of the unit, going as far as denying its actual existence, despite an NHK documentary drawing local and international attention to the travesties and the names and positions of hundreds of those working at the unit being officially released recently.
To accelerate Japan's surrender in the WWII, the US forces dropped two atomic bombs on Hiroshima and Nagasaki respectively on Aug. 6 and Aug. 9, 1945. Japan surrendered to the Allied Forces on Aug. 15, 1945, bringing an end to WWII.