Matsui said the number of hibakusha alive today is decreasing, and therefore "listening to them grows ever more crucial".
Mayor Kazumi Matsui opened his address at the annual ceremony by describing the scene on August 6, 1945, and the agony of the victims, telling the audience to listen "as if you and your loved ones were there".
In his speech this year, Abe said Japan will "make strenuous efforts to serve as a bridge between nuclear powers and non-nuclear states".
About 50,000 people, including Hiroshima residents and representatives from 80 countries, including U.S. Ambassador William Hagerty, attended this year's ceremony.
And less than two months ago, U.S. President Donald Trump and North Korean leader Kim Jong Un agreed to the "complete denuclearization of the Korean Peninsula" during their summit in Singapore, though the deal has been criticized for lacking detail.
Prime Minister Shinzo Abe, however, did not refer to the treaty in his speech to the ceremony, repeating the posture he took in last year's event held shortly after the treaty was adopted at the United Nations headquarters in July 2017.More news: Woman Helps Man Short On Cash, Then Finds Out He’s Keith Urban
Japan should take a more constructive role to achieve a nuclear-free world, he said, urging Tokyo to help the Treaty on the Prohibition of Nuclear Weapons take effect. For the vote of the treaty text, 122 nations voted in favor while 69 nations chose not to vote - all of which were nuclear weapon states.
"Atomic bombs were created by the evil ideas of human beings", said Sunao Tsuboi, the 93-year-old head of one of the groups of those who survived the US atomic bombing of Hiroshima on August 6, 1945, in the closing days of World War II.
Without naming specific nations, Matsui warned that "certain countries are explicitly expressing self-centered nationalism and modernizing their nuclear arsenals".
This computer graphic image provided by Fukuyama Technical High School shows a burning building just after an atomic bomb fell in Hiroshima, western Japan. In order to gain cooperation from both sides, it is important for everyone to understand "the reality of the tragedy of nuclear attacks", he said, reiterating Japan's pledge to maintain its pacifist and non-nuclear principles.
Matsui's call however highlighted Japan's contradictory relationship with nuclear weapons although Japanese officials routinely argue that they oppose atomic weapons.
The US attack on Hiroshima ultimately killed more than 140,000 people; the bombing of Nagasaki ended another 70,000 lives three days later.