France is now in the fourth week of its biggest youth protest movement in decades, with hundreds of high schools blockaded as students demand the replacement of absent teachers and repairs to decrepit buildings. In response, French lawyers and rights groups say teenagers have suffered war-like injuries from police weapons, and an unprecedented number of children have been taken into police custody - some reporting being hit or racially insulted.

Interior minister Laurent Nuñez told the senate that police have opened internal investigations into 36 allegations of officer violence against demonstrators, including four children allegedly "mutilated" by explosive weapons and six sustaining "significant injuries," plus complaints of physical blows, insults, or alleged sexual assault.

Evan, 15, a carpentry student at a vocational high school, had a hand blown off by a police stun grenade outside a school protest in Lens, northern France, on 5 October. His lawyer, Bertrand Wattez, said: "His injuries resemble war injuries." Wattez said Evan had gone to school for class, discovered a protest under way, and did not take part in violence. He denied the Béthune state prosecutor's claim that the grenade had been picked up from the ground. When it exploded, Evan lost his left hand, several fingers from his right hand, and suffered injuries to other parts of his body. He remained in hospital "in a very difficult condition," and his family filed a criminal complaint.

Another boy, Eden, 14, near a school protest in Saint-Ouen-l'Aumône, north-west of Paris, suffered extensive facial injuries: two fractures to his jaw, the loss of his bottom teeth, and all his top teeth broken after he was allegedly hit by a shot from a grenade launcher. His lawyer, Elsa Marcel, said the shot was fired at close range, from 1.5 metres. "This is the equivalent of war weaponry," she said. The family filed a legal complaint for attempted murder. Eden's parents said he found the school blockaded and wanted to go home. Video footage appeared to show him facing three riot police officers; one officer hit out with a truncheon, Eden kicked in the direction of police, and then a grenade shot was fired. An officer has been suspended and placed under official investigation.

This week, a police officer in the western city of Tours was charged with aggravated battery after Maël, 16, a vocational high school student, lost the use of his eye allegedly after a shot from a form of rubber-bullet launcher. The officer denies the charges. The family's lawyer said Maël had not been involved in any violent confrontation but had simply been present at a school protest, and the family felt a sense of "injustice."

France has faced scrutiny for years over police use of explosive weapons, particularly after serious injuries during the 2018-2019 gilets jaunes (yellow vests) demonstrations, including allegations that people lost eyes and had hands and feet mutilated.

The current protests have also led lawyers to raise concerns about an unprecedented number of children arrested and placed in police custody, with some reporting being hit, humiliated, or verbally abused during arrests. Since the school protests began in mid-September, a total of 6,773 under-18s have been taken into police custody after arrest, the justice ministry said. For more than 1,000 of those children, no charges were brought; 2,190 face legal action, and almost 2,000 were handed an alternative to further legal action, such as a citizenship course, without their case being heard at trial and without a conviction. Lawyers said accusations against children included gathering with violent intent, violence against police, and damage to property.

Nathalie Tehio, a lawyer and president of the French Human Rights League, said that only a minority of children's cases ending up before a judge raised concerns that arrests could be unfounded. "There are a record number of arrests … Often, the legal dossiers are empty. In reality, this is more about stopping people from demonstrating," she said. "I think there is a sense of hitting hard to try to dissuade people and scare them." Tehio added: "We have received accounts of racist comments made. Lawyers have told us that their clients say they have been humiliated or treated violently or pushed."

Romain Boulet, co-president of France's Association of Criminal Lawyers, said: "Criminal lawyers are worried and raising concerns about the potential misuse of police powers to dissuade citizens aged under 18 from demonstrating."

Mohamed Jaite, president of the Paris branch of the leftwing union Syndicat des Avocats de France, has represented several teenagers stopped in Paris and its suburbs. "I have seen one youth limping in a police station," he said. "Some have reported being humiliated or receiving racist and sexist insults."

Prisca Ancion, a lawyer from the leftwing lawyers' group Collectif d'Action Judiciaire, which demonstrated this week in front of a high school in Saint-Denis, in the northern suburbs of Paris, said: "There have been racist insults during arrests, as well as homophobic, sexist comments or sexual touching. We encourage families to file complaints." Another lawyer from the collective, Romane Bartoli, who assisted several teenagers after their arrest, said: "Youngsters told us they were slapped, kicked or punched."

One 16-year-old from a high school in a suburb north of Paris described being arrested 100 metres from his school on the day of a blockade in late September. He plans to study medicine at university and said he had intended to go to class before he saw there was a protest. Answering questions from the Guardian with his lawyer, he said he had been held in police custody for more than 12 hours, mostly with two others in a police cell with two mattresses. "It was hard psychologically because I felt I hadn't done anything wrong," he said. "I was hit with a truncheon and punched." He said a police officer told him to "shut his mouth" and said: "It's always you lot," which he understood to be a racist insult. He was released after a state prosecutor instructed him to carry out a citizenship course as an alternative to facing legal action.

The justice ministry and the interior ministry, which oversees policing, did not comment on allegations of insults or violence in police custody. In parliament, Nuñez denied any form of excessive policing or police repression. He suspended the use of stun grenades at school demonstrations.

Audrey Bailleul, a magistrate and national representative of the Union Syndicale des Magistrats, said it was hard to have a clear view of events at this stage. "There is nothing to show that there has voluntarily been a massive intention to not respect [legal] procedure," she said.