Mayor Itcho Ito blamed countries with nuclear arms, including the U.S. and North Korea, for not working earnestly toward nuclear disarmament.
Ito also said that an international pact to prevent nuclear proliferation was "on the verge of collapsing."
Participants in the service at Peace Memorial Park, just a few hundred meters (yards) from the center of the blast, observed a moment of silence at 11:02 a.m., when the bomb was dropped 61 years ago.
Kikuyo Nakamura, 82, said she was concerned that many of those who survived the bombing had died and that the atomic attack could soon be forgotten, according to the AP.
Speaking at the ceremony, Prime Minister Junichiro Koizumi said Japan would honor its pacifist constitution which prevents it from seeking or developing nuclear weapons and uphold the principle of nuclear nonproliferation.
About 4,800 people attended the ceremony including embassy officials representing seven countries, according to city official Mika Matsuo.
Wednesday's ceremony followed a memorial service on Sunday in Hiroshima, where about 45,000 people gathered in the city's peace park.
KGB General Nikolai Leonov, who personally knew Lee Harvey Oswald, talks about the version of John F. Kennedy's assassination on the orders from Nikita Khrushchev