Several shops were damaged or looted in the French port city of La Rochelle following multiple rallies on Saturday against a local irrigation project.
At least one police officer and five protesters were injured as police attempted to disperse the crowd, using tear gas and water cannons. At the same time, protesters threw projects and launched fireworks at the security forces.
"We were in the demo, they started blocking ahead and behind," said 25-year-old Lilia was quoted as saying the AFP news agency.
"They isolated us off to one side to charge everyone else."
The protesters oppose the construction of massive irrigation reservoirs in La Rochelle. They claim the reservoirs are aimed at helping big farmers and agricultural companies at the expense of smaller producers and the environment.
Some 2,000 took part in a march that was charged by police and dispersed. Running battles ensued and barricades were erected.
A second, more peaceful march of some 3,000 people, including family groups, marched from the city center toward the site.
Defenders of the project say reservoirs will save farms as Europe faces increasingly severe droughts.
In a separate incident earlier on Saturday, some 200 demonstrators, including farmers on tractors, entered La Pallice port terminal at dawn and set up a street party with music and drinks. They were pushed out by the police, while other vehicles blocked access to the port.
Some of the protesters used kayaks to approach the site from the sea.
The crowd on Saturday included small regional farmers and environmental activists, with a police source telling the AFP news agency that some 400 participants had joined a so-called "black bloc" of far-left radicals.
Several people, including police officers, were injured and seven protesters were arrested.
What did authorities say?
"A number of radical, extreme left-wing participants have attacked property and rampaged a supermarket," Interior Minister Gerald Darmanin said in an online post.
"It is hard to see a link to defense of the environment," he added.
He also said that police prevented the radical protests from achieving their objectives, but did not specify what those goals were.
The organizers decided to go ahead with their demonstration, despite an official ban.
A member of the controversial Soulevements de la Terre (Uprisings of the Earth, SLT) group, which the French government tried to ban last year, rejected that the protesters had violent intentions.
On Saturday, "our aim wasn't to clash with law enforcement, it's often law enforcement who aim to clash with us," SLT member Juliette Riviere said.
The intense police response was reminiscent of scenes from a similar protest at the Sainte-Soline reservoir last year that left two protesters in comas.