Japanese citizens protested Monday against the harbouring of nuclear-powered submarine La Jolla at the US naval base in southern Sasebo city, Nagasaki province, reported dpa.
More than 40 people staged a sit-in at the peace memorial in Nagasaki's capital, which was destroyed by the 1945 atomic bombing. The city is scheduled to mark the 63rd anniversary of the devastation on August 9.
Protests have mounted since another US nuclear-powered submarine, the Houston, was found to have leaked a small amount of radioactivity at the same Sasebo port in March.
The Japanese Foreign Ministry only reported the radioactive leak on Saturday - after news had filtered out - drawing sharp criticism from citizens' groups.
"The impact of radioactivity cannot be overestimated no matter what, even though the US military and Japan's Foreign Ministry were just trying to hide it," Kyodo News Agency quoted Koichi Kawano, 68, vice chairman of the Japan Congress Against A- and H-Bombs.
La Jolla, a Los Angeles-class fast attack sub like Houston, called at Sasebo base to give its 138 crew members a rest in Japan. A fire broke out onboard La Jolla in July 2004, when it was at Sasebo.