South Africa's police watchdog is investigating the death of a taxi driver after officers allegedly tied him to the back of a police van and dragged him through the streets.

Mido Macia, a 27-year-old Mozambican immigrant, was later found dead in a Daveyton police cell, on the eastern outskirts of Johannesburg.

Video footage has emerged of the incident, which has shocked the country and sparked outrage.

At least three policemen are alleged to have participated in the incident.

A murder investigation has been opened, according to Moses Dlamini of the Independent Police Investigative Directorate.

The police watchdog agency said the incident started just before 7pm on Tuesday when the taxi driver was allegedly obstructing traffic with his vehicle.

Mr Macia then allegedly assaulted a constable and took his weapon before he was overpowered.

The footage shows a man being dragged along the road by the vehicle at slow speed.

He tries to keep step even though he is almost horizontal above the ground.

Then the van stops and two policemen pick up his legs and drop them to the ground as the van picks up speed and drives off, beyond the view of the camera.

The watchdog agency said Mr Macia was found dead in a cell over two hours later by another policeman.

Evidence shows that he suffered head and upper abdomen injuries, including internal bleeding, they added.

National Police Commissioner Riah Phiyega has "strongly condemned" what happened.

Commissioner Phiyega urged South Africans to "remain vigilant and continue to report all acts of crime irrespective of who is involved".

The Daily Sun, a South African newspaper, posted the footage online and it was quickly picked up by other media.

The video evidence of the abuse renewed concerns about brutality, corruption and other misconduct by a national police force whose reputation has suffered in recent years amid reports that many officers lack training.

Jacob van Garderen of Lawyers for Human Rights said hardly a week goes by without such stories of brutality.

"As horrific as it is, it is not exceptional," he added.