Thousands of demonstrators ignored a ban on the gathering to turn out for the protest at the site of the A-69 motorway between Castres and Toulouse in the southwest.
A police officer hit by a Molotov cocktail, two paramilitary gendarmes and two protesters were injured during the clashes, said the regional authorities.
Ecologists and left-wing activists are trying to stop the building of an extension of the motorway, which scientists say will destroy wetlands, farmlands, trees and underground water sources.
The first clashes broke out at around 3:00 pm (1300 GMT) when "radicalised protesters... determined to commit acts of violence" tried to reach a main road, said a statement from local officials.
Riot police used tear gas to try to disperse the demonstrations and keep them from the road leading to the construction site. Some of the protesters fired fireworks and hurled Molotov cocktails at security forces.
The prefecture of the Tarn region denounced "extremely violent attacks with catapults" and said the crowd contained "1,200 radical individuals".
They had "clearly come to cause damage with no link to the issue" of the motorway, said a statement from the prefect, Michel Vilbois.
"We have a duty to step in," said one activist from climate campaign group Extinction Rebellion. "As long as we are there, the motorway will not go ahead," she added.
Organisers of the protest near the village of Puylaurens, which was called by the Roue Libre group, said 7,000 activists had turned up. The prefecture put the figure at 1,600.
Some 1,600 police and gendarmes were deployed to deal with the protest.
The project, started at the beginning of 2023, has the backing of most local elected officials and is due to come into service at the end of 2025.
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