Yukio Hatoyama, an ex-prime minster of Japan, joined a weekly anti-nuclear protest outside current Prime Minister Yoshihiko Noda's office on Friday.
The action takes place during a week in which a second nuclear reactor was restarted, despite concerns that it sits on an active fault line.
Hatoyama told protesters, "I regret that politics has strayed far from the people's wishes."
"We must protect the new trend of democracy you represent. We must stop the restart of nuclear power plants," the 65-year-old politician, who served until June 2010, said.
Anti-nuclear sentiment is extremely high in Japan with regular actions against nuclear power. On Monday tens of thousands rallied in Tokyo in an anti-nuclear protest.