Nagasaki Mayor Tomihisa Taue said the accident at the Fukushima Dai-ichi plant, which was crippled by a tsunami last March, has exposed the risk of nuclear technology.
Taue urged Japan to make concrete plans to achieve a nuclear-free society and called for renewed commitment to a global ban on nuclear weapons.
"Many people in Fukushima still live in fear of radiation effects," Taue said.
Prime Minister Yoshihiko Noda renewed his promise to seek a society less reliant to nuclear power in a mid- to long-term policy platform due out within weeks.
"We will compile an energy structure that would reassure the safety of the people," he said.
Japan is compiling its energy policy for the next two decades. Noda's government is making a decision on how much Japan should reduce the share of nuclear energy by 2030.
Within a three-day period in 1945, the U.S. dropped two atomic bombs on Japan, first on Hiroshima, killing 140,000 people, and then on Nagasaki, killing 70,000. The attacks prompted Japan's World War II surrender.
The United States sent its ambassador, John Roos, to Nagasaki for the first time this year, a year after his deputy attended the ceremony.