DAVEYTON, South Africa - South Africa's police watchdog is investigating the death in custody of a Mozambican taxi driver who was filmed being dragged behind a police vehicle, a spokesman said on Thursday.

Video footage taken by a bystander shows 27-year-old Mido Macia handcuffed to the back of a police van and dragged through the streets, in front of a large crowd of shocked bystanders.

Half a dozen uniformed and police officers clad in stab vests -- with at least one brandishing an unholstered pistol -- can be seen circling the scene.

Local media reported that police beat up the taxi driver in Daveyton, east of Johannesburg, accusing him of parking his vehicle incorrectly.

He was later taken into custody, where he died.

"We are investigating an incident involving the death of man, allegedly at the hands of the police. We are shocked by the footage which has been released," said Independent Police Investigative Directorate spokesman Moses Dlamini.

"The circumstances surrounding his death are still allegations... let's find out what really happened," he said.

He said there were reports the man had tried to disarm a police officer before the attack.

The beating allegedly continued inside the police holding cells before his death.

Police commissioner Riah Phiyega expressed "deep concern" about the incident.

"The matter is viewed by the National Commissioner in a very serious light and it is strongly condemned," a statement said.

Public outrage

The incident, which is thought to have taken place on Tuesday, has caused outrage in South Africa.

It is just the latest in a series of crises to hit the beleaguered police service, which was pilloried for the shooting dead of 34 miners on one August day and for its handling of the Oscar Pistorius case.

"This appalling incident involving excessive force is the latest in an increasingly disturbing pattern of brutal police conduct in South Africa," said Noel Kututwa, Amnesty International's southern Africa director.

The Independent Police Investigative Directorate received 720 new cases for investigation of suspicious deaths in custody or in other policing contexts from April 2011 to March 2012 according to Amnesty.

The opposition Democratic Alliance party called for a full investigation and for the officers involved to be suspended.

"The fact that it was police who were the vigilantes in this case shows that we cannot trust the (South African Police Service) to uphold the law," said provincial parliamentarian Kate Lorimer.

"The fact that the crowd watched and did nothing to help, some even cheering, is a sad indictment of the state of our society."

Dlamini, of the police watchdog, said only the police department had the power to suspend the police officers involved in the incident.

"We can only investigate and recommend suspension, we have no power to say that they should be removed from their jobs," he said.

The police department did not confirm if the officers had been suspended.