Organizers and police say that some 20,000 people turned out for Switerland's biggest anti-nuclear power demonstration in a generation.
Demonstrators on Sunday marched peacefully near the Beznau commercial nuclear power plant, the oldest in Switzerland, in the northern canton (state) of Aargau. It started operating 40 years ago.
The last such major demonstration in Switzerland drew 18,000 people in 1986 against the Goesgen nuclear power plant in the northern Swiss canton Solothurn.
A spokeswoman for the activists Michaela Loetscher said Japan's Fukushima Dai-ichi nuclear plant that was crippled by the March tsunami "has entirely changed the energy debate" in Switzerland.
Five nuclear power plants supply about two-fifths of the nation's energy needs.