Police in South Africa detained more than 30 people overnight as xenophobic violence simmered around the economic hub of Johannesburg, officials said on Saturday.

Anti-foreigner violence that erupted in the country's eastern port city of Durban several weeks ago, leaving at least six people dead, has spread to Johannesburg, displacing thousands and sparking alarm at the United Nations and in neighboring countries.

Overnight, small groups attacked shops inseveral areas around Johannesburg, police said.