More than 30 people have been detained by police in South Africa after a fresh outbreak of anti-foreigner violence.

Overnight, small groups attacked shops in several areas around Johannesburg, the country's commercial hub.

Police used rubber bullets to disperse looters in Alexandra, a township north of the city.

At least six people have died in anti-foreigner attacks in the eastern port city of Durban, with violence spreading to other parts of the country.

Several thousand foreigners have fled their homes to shelter in makeshift camps, and neighbouring Zimbabwe, Malawi and Mozambique have announced plans to evacuate citizens.

Meanwhile, police have urged people to stop sending unverified social media messages about attacks on foreigners because they are causing panic.

President Jacob Zuma has cancelled a trip to a summit in Indonesia because of the wave of violence against foreigners. On Saturday afternoon he is due to visit the Chatsworth refugee camp in Durban.

'Undercut by immigrants'

Addressing the latest violence, police spokesman Lungelo Dlamini told the AFP news agency: "More than 30 people were arrested last night. They are going to be charged for public violence, malicious damage to property, house breaking and theft."

Overall, 50 people have been arrested in the Johannesburg area and 112 in Durban since the trouble began.

Migrants, mostly from other African states and Asia, have moved to South Africa in large numbers since white-minority rule ended in 1994.

Many South Africans accuse them of taking jobs in a country where the unemployment rate is 24%.

resident Zuma has condemned the xenophobic attacks as "shocking".

"No amount of frustration or anger can justify the attacks on foreign nationals and the looting of their shops," he told parliament on Thursday.

Zulu King Goodwill Zwelithini has been accused of fuelling the attacks by saying that foreigners should "go back to their countries". However, he says his comments were distorted.

Official data suggests there are about two million foreign nationals in South Africa, about 4% of the total population. But some estimates put the number of immigrants at five million.

Many South Africans are against the violence, but are also unhappy with the level of immigration and feel they are being undercut by immigrants from poorer countries, the BBC's Milton Nkosi in Johannesburg reports.

At least 62 people died in xenophobic attacks in South Africa in 2008.