A silent prayer was held in the Japanese city at 8:15 on Wednesday morning – the same time as an American plane dropped the four-ton uranium bomb on August 6, 1945.
About 140,000 of the city's inhabitants are estimated to have been killed immediately or as a result of radiation injuries in the following years.
Warns of arms build-up
Hundreds of people, including survivors and relatives of those who lost their lives, laid down flowers during a ceremony at the memorial after the prayer.
Hiroshima's mayor Kazumi Matsui warned in a speech of "an accelerating trend of military build-up around the world".
He further stated that the international community blatantly ignores the lessons that should have been learned from the tragedies of history.
"A world without nuclear weapons"
Three days after the bomb on Hiroshima, the USA dropped another atomic bomb over Nagasaki, which instantly annihilated over 74,000 people.
Japan's Prime Minister Shigeru Ishiba said in a speech during Wednesday's ceremony that it is Japan's mission to take the lead for "a world without nuclear weapons".
On August 6, 1945, the American plane Enola Gay dropped the four-ton uranium bomb "Little boy" over Japanese Hiroshima.
A heat wave of up to 4,000 degrees – enough to melt steel – hit the city and its 350,000 inhabitants.
Three days later – during the last days of World War II – Nagasaki was also attacked with an atomic bomb called "Fat man", dropped from the plane Bock's Car.
The bombings took place on the orders of US President Harry Truman and are considered by many to have been the direct reason for Japan's capitulation in the war. In 1941, Japan had bombed the American naval base Pearl Harbor.
Truman had declared before the bombing that he wanted a quick end to the war by inflicting destruction on Japan and instilling fear.
The atomic bombs are estimated to have taken over 300,000 lives directly or indirectly through radiation injuries, for example through various forms of cancer.
Japan is the only country in the world that has been subjected to a nuclear attack.
