About 30 people, including policemen, were injured on Tuesday when police evacuated a camp preventing building work on the line, after a week-long demonstration.

Buildings in Turin's historical centre, recently scrubbed clean for the 2006 Winter Olympics which the city is hosting, were covered in graffiti saying "Hands off Val di Susa" and "No to the high-speed train, No to the Olympics."

Val di Susa links Turin to the Olympics mountain sites, where the games are due to begin on Feb. 10.

Prime Minister Silvio Berlusconi said extremist groups were behind the violence.

"Extreme left-wing groups, from antagonistic and anarchic-insurrectionist wings are trying to extend the unrest from Val di Susa to Turin, Rome, Milan, and various other cities," Berlusconi said in a statement after a meeting with Interior Minister Giuseppe Pisanu.

"The government is committed to dealing firmly with this plan, which has nothing to do with the peaceful protest by inhabitants of the valley."

The protesters blocked roads including a motorway connecting Turin with France. They later obstructed railway tracks, a spokesman for Italy's state railway company said.

Thousands of people have marched against the line, which will require building a tunnel more than 50 km (31 miles) long under the Alps.