Thousands of demonstrators crowded anti-nuclear rallies in Europe Saturday, donning gas masks in West Germany and blockading a nuclear power plant in the Netherlands on the first anniversary of the Chernobyl nuclear power plant disaster.

Protests turned violent in Switzerland, where police used tear gas to break up an illegal march, and in the Netherlands as police swinging batons dispersed club-wielding protesters outside an aluminum plant.

Yesterday was the anniversary of the April 26, 1986, explosion and fire that engulfed a reactor at Chernobyl poer plant in the Soviet Ukraine, releasing a cloud of radiation that traveled around the world. Soviet authorities did not publicize the accident for three days. Later they said that it killed 31 people and injured hundreds.

A protest organizer, Peter Zuber, said 18 people were hurt when Swiss police fired tear gas and rubber bullets to disperse thousands of protesters who broke away from the official march route in Bern. Demonstrators smashed bank windows, tossed firecrackers in the air and damaged a fence around a construction site, he said.

The disturbances broke out after demonstrators left an anti-nuclear rally which drew at least 10,000 people, according to estimates from police and demonstration organizers.

In the Netherlands, about 200 demonstrators with clubs tried to blockade the entrance to the Pechiney aluminum firm near the southern town of Borssele. Police mounted a baton charge, and at least one police officer and one demonstrator were wounded. No arrests were reported.

Police said the protesters came from a rally at a nuclear plant in Borssele, and apparently targeted the aluminum firm because of its heavy use of electricity produced by nuclear power.

At Borssele, authorities let hundreds of people conduct a weekend blockade of the plant's 10 gates. The protesters sat outside the plant's barbed-wire perimeter, many sunbathing as a brass band played.

In West Germany, demonstrators staged rallies and marches in Berlin, Bonn, Hamburg, Munich and Stuttgart.

Police said about 5,000 protesters marched through West Berlin's downtown area, wearing gas masks and carrying mock Geiger counters in a peaceful demonstration.

In Hamburg, however, a handful of protesters set fire to campaign posters for conservative parties in coming local elections. Police said an elderly passerby was treated at a hospital for shock. No arrests were reported.

The crowd in Hamburg, estimated by police at 6,000, waved banners reading "Remember Chernobyl" and "We Have Not Rid Ourselves of Chernobyl."

Thousands of British protesters paraded through London Saturday, led by Glenys Kinnock, wife of Labor Party leader Neil Kinnock. They observed a minute of silence as a siren sounded, symbolizing a nuclear warning.

Organizers estimated the crowd at just over 100,000, but Scotland Yard, the police headquarters, put the number at 48,000. Police described the march as peaceful and said there were only three arrests, all for drunkenness.