France's interior Minister says anti-government protests in France have "created a monster".
Christophe Castaner is warning that "radical elements" could infiltrate planned Yellow Vest protests this weekend.
Tourist sites in Paris are to close on Saturday amid fears of further street violence.
The government has defended riot police tactics after they forced dozens of detained high-school students to kneel in rows with their hands held behind their heads or in handcuffs after violent protests west of the capital.
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Students this week have blocked access to scores of high schools across France in protest at President Emmanuel Macron's education reforms, just as the 40-year-old leader grapples with sometimes-violent demonstrations over living costs.
Videos and photos of the students from two high schools in Val Fourre, a deprived neighbourhood outside Mantes-La-Jolie, 60km west of Paris, went viral on social media late on Thursday, prompting public outrage.
"Over the past few days, the students have been joined by about 100 hooded youths armed with clubs and incendiary devices and determined to pick a fight with police," Mr Castaner told a news conference.
Castaner said roadblocks had been set alight, projectiles hurled at motorists and houses robbed in the area around the two schools.
"It is in this context that the security forces stepped in," the minister added.
No students were injured while detained in the Val Fourre incident, French media reported. But some social media users said the scene, with some of the teenagers lined up facing a wall, resembled a mock mass-execution.
"Can anyone tell me if they've witnessed such a thing in the last 50 years," one Twitter user said.
Another tweet read: "These images of teenagers on their knees at the feet of [riot police] are unworthy of a democracy. The government needs to take charge and re-establish chains of command."
Education Minister Jean-Michel Blanquer described the images as "shocking" but said the violence convulsing France in recent weeks justified the heavy-handed policing.
France is hunkering down for another wave of potentially violent protests on Saturday as Macron struggles to quell public anger at the cost of living. Senior allies said he would address the nation early next week.