Amateur video footage of two armed uniformed policemen and private guards punching and kicking a man on a Cape Town street went viral last week.
The alleged victim has been identified as Nigerian national Clement Emekeneh.
“The video of the attack which has appeared on the social media is particularly disturbing,” Nigeria’s foreign ministry said in a statement, which described the incident as a “merciless beating.”
The Nigerian embassy in Pretoria has “sent a strongly-worded note to the Department of Foreign Affairs of South Africa protesting the attack and demanding that the perpetrators be brought to justice.”
In the video, a security guard ripps the shirtless man’s trousers off and then a police officer repeatedly punches and kicks the man in the groin.
Another officer holds the victim by the neck. His underpants come off as the beating continues before he is loaded into a police car.
Two officers, aged 42 and 45 years old, were arrested, charged with assault and granted bail on Monday.
The footage has sparked outrage among many South Africans.
South African police officers are frequently embroiled in allegations of brutality, yet prosecutions rarely occur.
Last year, Mozambican national Mido Macia, died after being dragged behind a moving police patrol van in a town east of Johannesburg. Nine officers are standing trial for his killing.
Xenophobic attacks across South Africa in May 2008 left 62 people dead and caused thousands to flee their homes.