An estimated 170,000 protesters marched through Tokyo's Yoyogi Park to protest the use of nuclear power in the wake of the March 2011 Fukushima nuclear crisis.
Japan last week restarted a nuclear reactor in Fukui and a second reactor begins the activation process later this week.
Anti-nuclear protests have been occurring weekly, The Japan Daily Press reported, but this one was by far the largest.
Demonstrators calling for an end to nuclear power also called for the resignation of Prime Minister Yoshihiko Noda.
Kyodo News said Nakoso Beach in Fukushima opened Monday. The beach, about 40 miles south of the damaged Fukushima reactor, is the first to open in the prefecture since the nuclear crisis. Local officials say low levels of radiation in the air and water pose no health risk.