Anti-foreigner violence that erupted in the country's eastern port city of Durban several weeks ago has so far left at least six people dead, spreading to Johannesburg, displacing thousands and sparking alarm at the United Nations and in neighboring countries.
Overnight, small groups attacked shops in several areas around Johannesburg, police said.
"More than 30 people were arrested last night. At this stage the situation is calm but we plan to increase our deployment," police spokesman Lungelo Dlamini told AFP.
Police had to use rubber bullets to disperse the looters in Alexandra, an impoverished township north of the city, he said.
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Several thousand foreigners have fled their homes to shelter in makeshift camps amid the violence, and neighboring Zimbabwe, Malawi and Mozambique announced plans to evacuate their citizens.
The latest burst of anti-foreigner violence in the country has been largely blamed on a speech last month by King Goodwill Zwelithini, traditional leader of the Zulus, in which he blamed foreigners for South Africa's high crime rate and said they must "take their bags and go".
This is not the first wave of anti-foreigner violence in South Africa. In January, foreign shopkeepers in and around the vast township of Soweto, south of Johannesburg, were forced to flee and six were killed as looters rampaged through the area.
And in 2008, 62 people were killed in xenophobic violence across the city's townships.