According to Sylvester, he got out of his car and was surrounded by officers, at least two of whom were carrying automatic weapons. Without warning, one officer fired a Taser into the back of his head which made him drop to his knees, he said. A second shock caused him to fall on his face, breaking a front tooth. A further six shocks made him wet himself and left him lying in the road in pain while the officers and sniffer dogs searched the car and found nothing.

The Independent Police Complaints Commission has started an investigation and David Lammy, Sylvester's MP in Tottenham, north London, has written to Sir Ian Blair, the Metropolitan police commissioner, to say he is "deeply concerned".

The incident was part of Operation Neon, a crackdown on guns on London's streets by using armed response units to stop and search cars. Sylvester said the incident had left him traumatised and he now suffered from short-term memory loss. He doubts the police would have stopped him had he not been black. A spokesman for the Met said: "Just after midnight, officers on an intelligence-led operation stopped a car in Bounces Road, N9. The driver got out of the vehicle and was subsequently Tasered. Our information is the Taser was deployed once."

Sylvester had been followed by police cars for about three miles through Tottenham before they boxed him in.

"Armed police jumped out and opened my car door," he said. "I said OK, I'm coming. I asked what was going on and as soon as I stepped out of the car I felt something touch me on the back of the head and then I was on my knees. Then it happened again and I was on my face and I felt somebody pressing my head down with their foot. By the fifth time I realised officers were pinning my arms together. It was like they were trying to break my arms and I was in pain, screaming out.

"I was shocked eight times altogether and I had urinated on the floor. It was like being tortured. It went on and on and I felt they were going to kill me."

According to guidelines set by the Home Office and the Association of Chief Police Officers, Tasers should be deployed "where officers are facing violence or threats of violence of such severity that they would need to use force to protect the public, themselves and/or the subject(s) of their action". Tasers have been used 47 times in London this year, with black people accounting for almost two-thirds of those stunned.

The government extended the right to use Tasers for all firearms officers in England and Wales this summer.