About 6,000 people joined initially peaceful marches in the Val di Susa area, near Turin, police said, protesting against a tunnel they say will damage the environment and spoil the picturesque Alpine valley.

But scuffles broke out at several points along the boundaries of the construction site. Protesters threw stones and firecrackers and police responded with tear gas.

At least 188 police officers were injured in the clashes, while several protesters and a site worker were also hurt. Several people were injured in similar clashes earlier this week.

Police arrested at least five people and attempted to disperse hundreds of protesters near the enclosure, although some managed to breach the site fences.

Italy's President Giorgio Napolitano released a statement condemning aggression against the police and said violent infiltrators of protests would not be tolerated.

Villagers in the valley strongly oppose the 15-billion-euro project, which is supported by the government and the local administration and backed by EU funding. France and Italy signed a deal to build the link in 2001.

Opposition has spread beyond the local area, winning support from a wide range of groups, including anarchists and Catholics.

Police said many of the people involved in the clashes on July 3 were exploiting the rally as an opportunity for violence and several had come from abroad.