Eight South African policemen have been arrested on suspicion of murder after dragging a man from the back of a police van through a busy Johannesburg street, a government watchdog has said.

The incident was caught on video and has further blackened the reputation of the police.

Moses Dlamini of the Independent Police Investigative Directorate said the officers will "answer to a charge of murder when they appear in court on Monday".

Mido Macia, a 27-year-old taxi driver from Mozambique, was found dead in detention with signs of head injuries and internal bleeding, according to an initial post-mortem report released by the police watchdog.

"We would like to assure the country and the world that what is in the video is not how the South African Police Service in a democratic South Africa goes about its work," Commissioner Riah Phiyega told a news conference.

South African President Jacob Zuma and opposition politicians have condemned the incident, which happened on Tuesday and was broadcast across the world yesterday.

"The visuals of the incident are horrific, disturbing and unacceptable. No human being should be treated in that manner," Mr Zuma said of what he called "the tragic death of a man in the hands of the police".

Police said they had detained Mr Macia after he parked illegally, creating a traffic jam, and then resisted arrest.

The video shows the man scuffling with police, who subdue him.

He is then bound to the back of a pick-up truck by his arms, before the vehicle drives off in front of dozens of witnesses in the east Johannesburg area of Daveyton.

It was the latest in a series of scandals to hit South Africa's police force in recent months.

The lead detective in the murder case against Olympic and Paralympic runner Oscar Pistorius was removed from the investigation last week, when it emerged he was facing seven attempted murder charges for allegedly opening fire on a minibus full of passengers.

Police shot dead 34 striking workers at a platinum mine in August last year, which was the deadliest security incident since apartheid ended in 1994.

The latest video and the taxi driver's death have raised fresh concerns about police brutality in a country where more than 1,200 people a year die while in custody.