Nigeria has urged “urgent” diplomatic action after one of its citizens died in a violent confrontation with South African police, in what they called a “barbaric” and “extra-judicial” killing.

The incident involves Nigerian man Victor Tochukwu Nnadi, who was allegedly choked as he lay handcuffed on a main street on Thursday.

Photos and film footage taken by witnesses show a prone body with a swollen face and blood coming from his mouth.

“The barbaric behaviour of the perpetrators is not only unacceptable, but also calls for urgent attention by diplomatic authorities in Nigeria and South Africa,” said Abike Dabiri-Erewa, a senior aide to the president on foreign affairs and the Nigerian diaspora.

“While appealing to Nigerians to avoid crimes, the extra-judicial killing of Nigerians is… unacceptable,” she said.

South African police said Nnadi resisted arrest on suspicion of drug dealing and died after swallowing a dose of heroin that he was trying to sell.

A group representing expatriates, the Nigerian Union in South Africa (NUSA), complained there had been no examination to confirm he had died of heroin.

South African police say they are carrying out further investigations.

“The accused is innocent until proven guilty,” NUSA spokesman Emeka Ezinteje Collins told AFP. “This is not a crime to be a Nigerian. We tend to believe that Nigerian lives don’t matter to them.”

Nnadi’s brother, who tried to prevent the arrest, was himself detained and remains incarcerated.

According to NUSA, there are around 800,000 Nigerians in South Africa, many of them living in Johannesburg.

The community was badly hit by a wave of xenophobic violence in April 2015 that, according to an official toll, left seven people dead and thousands homeless.

In its latest report, an independent watchdog said 640 people had died from police brutality or in police custody in South Africa.

AFP