Footage of the arrest in north London on Thursday shows two officers holding down the handcuffed man, Aljazeera reported.
At the beginning of the clip, one of the officers, who was white, appears to be applying pressure to the man's neck with his knee. The officer's hand is on the head of the man, who is on a pavement on his side.
Another officer of East Asian ethnicity assisted in the restraint.
"Get off me ... get off my neck," the struggling man can be heard shouting in the video that appeared on social media. "I haven't done anything wrong, get off my neck."
One of the two officers asks, "Are you going to behave yourself?" He is told to stay down.
The suspect was later shown sitting and speaking to the police officers.
Police said he was arrested on Thursday on suspicion of affray and possession of an offensive weapon, following reports of a fight, and remains in custody.
The Metropolitan Police referred the incident for investigation by the UK's independent police watchdog, and London Mayor Sadiq Khan called for a swift and thorough investigation of what he described as a "distressing" incident.
"The video footage that I have seen today and is circulating on social media is extremely disturbing," Metropolitan Police Deputy Commissioner Steve House said in a statement on Friday.
"Some of the techniques used cause me great concern - they are not taught in police training."
Major cities around the world, including London, have seen large Black Lives Matter protests over concerns about racial injustice following the death of George Floyd, an unarmed Black man in police custody in the US city Minneapolis on May 25.