Mr. Khan, the London mayor, said on Twitter that he was “deeply concerned” by the video and supported the decision to refer the matter to the independent police watchdog.
The police identified the man in the video as Marcus Coutain, 45, who was detained after reports of a fight in the area because he fit the description of a suspect and was carrying a knife. Mr. Coutain appeared in court on Saturday.
His lawyer, Timur Rusten, said at the hearing that the officers’ actions during the arrest mirrored “almost identically what happened to George Floyd.” He said that Mr. Coutain had been carrying a knife to fix his bike and that the police should apologize for his treatment.
Ben Bowling, a professor of criminology and criminal justice at King’s College London who has studied policing in Britain for 30 years, said that while abusive techniques were not new, it was “astonishing to see a police officer, with his knee on a man’s neck when he’s in handcuffs and on the ground, just weeks after the death of George Floyd in the U.S.”
“Power is insufficiently restrained in policing London,” Mr. Bowling added. “It is a pattern that is affecting Black Londoners, and it’s unacceptable.”
Handcuffing practices by the police in Britain came under the spotlight this month after Bianca Williams, a top British athlete, filmed officers who had stopped her and her partner, Ricardo dos Santos, a Portuguese sprinter, in West London. Ms. Williams, 26, and Mr. dos Santos, 25, who are both Black, were ordered out of their car and handcuffed in front of their 3-year-old son.
The encounter, during which Ms. Williams and Mr. dos Santos were found to have done nothing wrong, fueled anger at police tactics that were said to disproportionately target Black people in Britain.