Nigeria has protested to South Africa about a "merciless attack" by police on one of its nationals in Cape Town, its foreign ministry has said.
An amateur video on social media sites purportedly shows the man being stripped, assaulted and handcuffed by policemen and security guards.
Nigeria had sent a "strongly worded" note to Pretoria, demanding justice for the man, the ministry said.
Two South African police officers have been arrested.
Rights groups have often accused South African police officers of brutality, incompetence and corruption.
"The Nigerian High Commission will continue to monitor the case with keen interest until justice is achieved," the ministry said in a statement.
South Africa's police watchdog, the Independent Police Directorate, has said the officers would be charged with assault.
About 17,000 registered Nigerians live in South Africa, but correspondents say there are many more illegal immigrants.
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.