The images are grainy, but the content is clear. Less than three minutes of video, shot on a mobile phone and posted online by a newspaper in South Africa, has sent fresh shockwaves through a nation already reeling from the Oscar Pistorius murder case.

Warning: Graphic content in video

The video begins innocently enough: an argument between a police officer and a taxi driver who is causing an obstruction with his minibus in Daveyton, east of Johannesburg. But it quickly descends into a physical altercation, and the crowd of onlookers begins to whistle and yell. And as the uniformed police officer, joined by his colleagues, handcuffs the slender man in a red T-shirt to the back of a police van, his hands outstretched behind his head, curiosity turns into alarm with shouted warnings in Zulu: "We are going to film this!"

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Despite the audience, the police officers continue to torment the young man, picking him up by the ankles as the police vehicle pulls away, then dropping him as it accelerates. Their victim is still attached, his legs dragging along the Tarmac.

Just over two hours later the taxi driver, Mido Macia, 27, a Mozambican migrant, was found dead in a police cell. The newspaper which initially published the footage, The Daily Sun, says that others detained with him claimed he was beaten in custody.

South Africa's police watchdog, the Independent Police Investigative Directorate (IPID), announced it had launched a murder investigation after a post mortem examination revealed that Mr Macia died after suffering injuries to his head and upper abdomen, as well as internal bleeding. The officers involved claim that Mr Macia assaulted one of them, grabbing their gun, before being overpowered.

In a nation still coming to terms with the deaths of 34 platinum miners who were killed by police in Marikana, east of Pretoria, last year, the video was met largely with frustration and disgust. An inquiry into the shootings continues, while anger is still raw following allegations of heavy-handed police action against striking farmworkers near Cape Town this year.

On social networks South Africans labelled the Daveyton police "animals", while others mused how little has changed since the end of apartheid. South African President Jacob Zuma described the incident as "horrific, disturbing and unacceptable. No human being should be treated in that manner," he said.

Last night South Africa's Police Union said it was mortified by the video. "It tarnishes the good name of the men and women in blue who do work really hard, and serve the country. We are shocked, angry. If they're found guilty – they must face the full might of the law," spokesman Theto Mahlakoana said.

Amnesty International estimates that the IPID was called to investigate 720 new cases of suspicious deaths involving the police between April 2011 and March 2012. Rights groups and think-tanks are calling for a broader commission of inquiry into the actions of the South African Police Service. Cameron Jacobs, the South Africa Director of Human Rights Watch, said: "This is not the first time that we've seen acts of brutality or excessive force. It's also deeply concerning that this incident involved a foreign national. There are worries that this may have played a part."

Gareth Newham, who runs the governance, crime and justice programme at South Africa's Institute for Security Studies, says he's not surprised.

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"The illegal use of force happens daily. It's widespread, usually directed at vulnerable groups, and mostly unrecorded," he told The Independent. "They want to punish, not just restrain. They want to establish fear."

Newham's think tank estimates that there were more than 1.6 million arrests in South Africa last year and reports that civil claims against the police have doubled in three years.

He says South African police officers are told to be tough. "Take no nonsense, show no mercy, shoot to kill, they're told. And then, when they get away with it, it becomes routine." As The Independent went to press, a police spokesperson confirmed that the officers in question have not yet suspended.

Civil claims against the South African Police Service have doubled in the last three years

Law and disorder: The death toll

This is the latest of a series of high-profile cases of apparent police brutality in South Africa. First, 34 striking workers at Lonmin's Marikana mine were gunned down in August, in what was called the most deadly police action since the end of apartheid. Police said they were acting in self-defence, though the miners were said to have been unarmed. Later, police were accused of planting weapons next to the bodies of those killed.

Then, this month, after Oscar Pistorius was arrested on suspicion of killing his girlfriend Reeva Steenkamp, the detective in charge of the investigation, Hilton Botha, was found to be facing attempted murder charges. He is accused of firing at a minibus taxi carrying seven passengers in 2009, killing all on board. The charges had existed before the Pistorius bail hearings began, but accusations such as these against police are seen as so commonplace that the media paid little attention.

The process of demilitarising the police began when Nelson Mandela took power in 1994. But more recently, under Jacob Zuma, the country has seen a return to "tough policing". The police argue they are targets of violence, and around 100 officers are killed every year. But estimates say more than five times that number of civilians are killed by police each year. And little is being done to address the problem.