Mayor Tadatoshi Akiba said the United States worshipped nuclear weapons as "god" and blamed it for jeopardising the global nuclear non-proliferation regime.
"The Nuclear Non-Proliferation Treaty, the central international agreement guiding the elimination of nuclear weapons, is on the verge of collapse," Akiba said in an address to some 40,000 people.
The chief cause is US nuclear policy which openly declares the possibility of a pre-emptive nuclear first strike.
The US has also proposed resumed research into mini-nukes and other so-called "useable nuclear weapons", the mayor said. "It appears to worship nuclear weapons as god," he concluded.
"The Nuclear Non-Proliferation Treaty, the central international agreement guiding the elimination of nuclear weapons, is on the verge of collapse."
--Hiroshima Mayor Tadatoshi Akiba
The mayor also slammed as unjust the US-led invasion of Iraq, which he blamed for killing innocent civilians. "The weapons of mass destruction that served as the excuse for the invasion have yet to be found," he said.
Akiba urged US President George W. Bush and North Korean leader Kim Jong-Il to personally visit Hiroshima and "confront the reality of nuclear war".
During the 45-minute ceremony, officials added 5,050 names to the register of victims who died immediately or from the after-effects of radiation exposure in the bombing, bringing the total toll to 231,920, an official said.
The Hiroshima bombing was followed by the dropping of a second atomic bomb on the city of Nagasaki on August 9, 1945, which killed another estimated 74,000 people.
Prime Minister Junichiro Koizumi told the service that Japan would stick by its pacifist constitution and its non-nuclear principles because the tragedies of Hiroshima and Nagasaki "can never be repeated."