Theo Luhaka, who was 22 at the time, was left disabled after suffering severe anal injuries from a police baton, as well as wounds to his head and face, during the ID check in the Paris suburb of Aulnay-sous-Bois in February 2017.
A medical report in 2019 found his injuries would require life-long treatment.
Dressed in a blue puffa jacket, Luhaka sat in the first row of the packed courtroom in the capital's suburb of Bobigny on Tuesday.
Marc-Antoine Castelain, 34, Jeremie Dulin, 42, and Tony Hochart, 31, appeared in the dock in a rare case of alleged police brutality to be tried in a criminal court instead of at an internal disciplinary hearing.
FILE - French youth worker Theo Luhaka, arrives to address people during a rally against police violence organised by the "Justice for Theo" group, outside the courthouse of Bobigny, near Paris, on October 28, 2017.
Castelain has been accused of voluntary violence that inflicted a "permanent disability" and risks up to 15 years in prison.
His two colleagues are being tried for taking part in the assault, including over allegedly kneeing and punching Luhaka while he was in handcuffs on the ground.
The trial is to last 10 days, with a verdict expected on January 19.
'Brutality and racism'
Police oversight body IPGN concluded there had been a "disproportionate use of force" in the incident, and that the two violent baton blows were inflicted at a time when "Luhaka was not attacking the physical integrity of the police officers."
"I used a blow that caused (Luhaka's) injury that was taught to me at the (police) academy, a legitimate blow," Castelain told the court.
"I am aware that it is a serious injury. I want to reiterate my deep compassion" for Luhaka, he added.
"This case has meant I've lost everything. My home, my profession. I've been used as a scapegoat to avoid the suburb exploding," Castelain said.
FILE - This court sketch on January 9, 2024 at the Assize court in Bobigny, near Paris, shows the three police officers who appear, LtoR, Marc-Antoine Castelain, 34, Jeremie Dulin, 42, and Tony Hochart, 31, over the violent arrest in 2017 of French Theo Luhaka (3rdR), 28.
"We police officers are victims of this case in our own way," he added.
A video surveillance camera recorded how Castelain struck Luhaka when he and fellow defendant Dulin were pinning the young man against a wall.
The violent blow with the tip of his telescopic baton left the young man with a ruptured sphincter.
Castelain denied rape allegations from Luhaka, who has since become a symbol of rough policing tactics used against youths in the high-rise housing estates that ring Paris.
FILE - This court sketch made on January 9, 2024 shows police officer Marc-Antoine Castelain (C), 34, during the first day of the trial over the violent arrest in 2017 of Theo Luhaka.
Prosecutors meanwhile said there was not enough evidence to support the rape charge.
The video footage was shared widely on social media, causing an uproar and prompting then-president Francois Hollande to visit Luhaka in hospital.
Racism allegations
Activists have repeatedly accused French police of brutality and racism.
In July 2020, three officers were charged with manslaughter over the death of delivery man Cedric Chouviat, who was held in a chokehold during his arrest for a traffic offence.
But in September 2023, lawyers said investigating magistrates had dropped their case against three gendarmes over the 2016 death in custody of 24-year-old black man Adama Traore.
Meanwhile riots lasting several nights hit many French cities last summer over the death of 17-year-old Nahel M. in a police traffic stop outside Paris.
The teenager's mother was in court on Tuesday to support Luhaka's family.