Dozens of people have been arrested in the South African city of Johannesburg after rioters looted shops and torched buildings and vehicles.
Police fired tear gas, rubber bullets and stun grenades in an attempt to quell the unrest on Monday.
Many of the targeted shops are reported to be owned by foreign nationals.
But the country's police minister, Bheki Cele, said "criminality rather than xenophobia" was to blame for the "senseless violence".
"[Xenophobia] is used as an excuse," he told reporters after visiting the city's Central Business District, where much of the unrest has been taking place. "Nothing... has sparked any form of conflict between the South Africans and foreign nationals."
However, Nigerian Foreign Minister Geoffrey Onyeama said Nigerian shops had been targeted by "mindless criminals" and promised to take "definitive measures".
South Africa has seen a spate of xenophobic attacks in recent months. In March, three people died during protests that targeted foreign-owned shops in Durban.
The latest unrest reportedly began on Sunday when a building in Johannesburg's Central Business District caught fire and collapsed, killing at least three people.
On Monday, the violence spread to the nearby area of Jeppestown before reaching other areas of the city such as Denver, Malvern and Tembisa.
Hundreds of people marched through the Central Business District, where video shared on social media showed shops being looted and rows of burnt-out cars. At least 41 people were arrested after the march, police said.
A mix of thuggery and xenophobia
Analysis by Milton Nkosi, BBC News, Johannesburg
The looting of foreign-owned shops in South Africa is becoming a daily occurrence and there is no sign of it stopping soon.
Many people have been asking why black South Africans are attacking other Africans. There is no easy answer to this, but it does seem like a combination of straightforward thuggery and xenophobia.
It is important to see these events within its context. South Africa is home to many immigrants who never get attacked and there are many South Africans who get caught up in the senseless violence as well.
But the unrest here is largely an immigration issue. The unemployment rate in South Africa is nearly 28% and this places huge pressure on the lower end of the social ladder.
"We've stabilised the situation and arrested a few dozen people already," police spokesman Wayne Minaar told Reuters news agency on Monday.
"We can't confirm the final figure right now but they will be charged for public violence," he said. "There's also a charge of attempted murder being investigated."
The violence comes just days after similar unrest was seen in the capital, Pretoria.
There were clashes between police and rioters, and foreign-owned shops were reportedly looted and destroyed.