French authorities held 46,955 migrants in the detention centers across the country and in overseas territories in 2023, compared to 43,565 the previous year, a report by migrant rights groups including SOS Solidarity and France Terre d'Asile ("France Land of Asylum") said.
In mainland France, the large majority were men, 5 percent were women and 87 individuals were children accompanied by their parents. More than 120 said they were under-18 but French authorities found them to be adults.
Most were Algerian, Tunisian and Moroccan, in that order.
On average they were held 28.5 days out of a maximum allowed of 90 days, a week more than the previous year.
This had impacted the mental health of the detainees, sometimes leading to suicide attempts, self-mutilation, tensions and even violent incidents with people working with them, the report said.
Last year, four migrants died, it added.
"Never have our associations witnessed so many violent acts as in 2023," the report said.
It added that detainees sometimes clashed with others held with them, but some also complained of police violence.
For example, at one center in the Paris region, more than 40 migrants officially complained of "physical violence, threats or insults of racist or homophobic character, (and) sexual assaults" from police inside the facility, it said, adding others with similar stories likely did not come forward.
But of those held in detention centers, 15 percent fewer had been expelled from the country last year compared to 2022, despite an increase in deportations overall, the report said.
This contradicted "the political argument claiming that detention is the only way to allow removals," it said.
The reported noted that "since 2022, France has been responsible for more than a third of all removal measures issued across the European Union."