Eyewitnesses said the incident came after the anarchists marched from an anti-G8 campsite near the French-Swiss border city to blockade a building where French Socialist Party activists were meeting.

It was the first clash in what both protest leaders and police had said would be a series of peaceful marches and sit-ins during the three-day summit in the nearby French spa town of Evian, which opens today.

Authorities on both sides of the border, fearful of a repetition of violence that rocked a G8 summit in Genoa, Italy, two years ago, have mounted a massive security operation involving some 25,000 police and security officials.

It was not immediately clear why the anarchists, carrying their traditional black-and-red flags, decided to march against the socialists -- although hard-left movements generally regard social democratic parties as "hirelings of capitalism."

Marchers said they threw stones at the building where the socialists were meeting to discuss the G8 summit after guards inside fired a spray at them. There was no immediate comment from the socialists.

The anarchists began setting up barricades but French riot police moved in, firing volleys of teargas to push them back to the campsite at Annemasse airfield, where earlier in the day protesters were instructed in passive resistance techniques.

Police barricades were going up on both sides of the French- Swiss border around Lake Geneva to halt protesters trying to move off permitted routes. Army helicopters circled overhead.

In Lausanne, across Lake Geneva from Evian, various groups denouncing the G8 leaders, who include President Bush and Russian President Vladimir Putin, were considering ways to try to block water traffic.

While most G8 leaders are staying around Evian, about a dozen prime ministers and presidents who are summit guests, including China's Hu Jintao, will stay in Lausanne and travel across Lake Geneva from the Swiss to the French side.

Protesters accuse the G8 chiefs of acting like "Masters of the Universe" and say the poorer nation leaders -- who include Brazil's new left-wing President Luis Inacio Lula da Silva -- will get nothing more than crumbs from them.

The G8 include Britain, Canada, France, Germany, Italy, Japan, Russia and the United States.