Johannesburg - "The violent killing of a Daveyton taxi driver, allegedly by police officers, is senseless and barbaric and must be condemned in the strongest possible terms," Congress of the People spokesman Leonard Ramatlakane said in a statement.
"The police showed utter disregard for the rule of law by continuing to engage in their ruthless assault exercise."
Ramatlakane said police officers demonstrated a lack of recognition of human rights.
He called for the suspension of the officers who allegedly dragged a 27-year-old Mozambican man behind a police van on Tuesday.
Mido Macia was later found dead in the holding cells of the Daveyton police station.
National police commissioner Riah Phiyega was "deeply concerned" about allegations that police brutality led to Mido's death.
"The matter is viewed by the national commissioner in a very serious light and it is strongly condemned," said her spokesman Brigadier Phuti Setati.
He said the death had been reported to the Independent Police Investigative Directorate (Ipid) for criminal investigation, and the police would fully co-operate with its investigation to ensure justice was served.
Phiyega had also instructed acting provincial commissioner Maj-Gen Phumzo Gela to start an internal investigation into the death and provide feedback urgently, he said.
Amnesty International called the death shocking.
"This appalling incident involving excessive force is the latest in an increasingly disturbing pattern of brutal police conduct in South Africa," the organisation's Southern Africa director Noel Kututwa said in a statement.
"Amnesty International urges the South African government to make a public commitment to ensure that the police stop the use of excessive force and deliberate targeted killings."
On Wednesday, the Ipid confirmed that the death was being probed.
"We are still interviewing the witnesses because the allegations are that he was dragged (along) with the police vehicle," spokesman Moses Dlamini said.
He said two policemen switched on their siren, indicating that Macia should move his vehicle, when they saw it obstructing traffic in Empilweni Street on Tuesday.
"It is also alleged that the taxi driver then assaulted the constable and took his police firearm. The warrant officer overpowered the taxi driver and handed the firearm back to his colleague," he said.
The warrant officer then took a taxi to the Daveyton police station to call for back-up.
"When back-up arrived, the constable was still at the scene, struggling to put the suspect in the police van," said Dlamini.
He said officers claimed they put the "resisting suspect" into the police van and took him to the police cells.
He was found dead by another police officer later in the day.
An inquest docket had been opened, and the police had also opened a case of assault with intent to cause grievous bodily harm.
Dlamini said a post mortem report received on Wednesday indicated Macia died of head injuries, with internal bleeding.
"They (policemen) are only saying they locked him (up) and he was found by another police officer, but we are investigating," he said.
By Thursday afternoon no arrests had been made and Ipid was still collecting evidence.
"If we find evidence, the arrests will happen, even if it is not today or tomorrow," said Dlamini.
The Democratic Alliance called on the SA Human Rights Commission (SAHRC) to investigate.
"(This) shows the deep, systemic problems of brutality and lack of discipline and professionalism within (the SA Police Service) SAPS," said DA police spokeswoman Dianne Kohler-Barnard.
"How much longer must South Africans live in fear of the very people who are supposed to protect them?" she asked.
The Police and Prisons Civil Rights Union (Popcru) said it supported the Ipid's investigation.
"Popcru is mortified by these actions, which demonstrate the opposite of what the men and women in blue represent," spokeswoman Theto Mahlakoana said.
SAHRC spokesman Isaac Mangena said the death or assault of anyone in police custody deserved a full investigation. - Sapa