Mobs armed with knives, clubs and jugs of petrol torched shacks and beat migrant workers on Monday in escalating anti-foreigner violence in Johannesburg squatter camps.

The unrest has killed at least 22 people since last week and increased political instability at a time of electricity shortages, rising inflation and disaffection among the poor over President Thabo Mbeki's pro-business policies.

Police fired rubber bullets at rioters and police helicopters hovered overhead as officers rounded up suspects involved in some of the worst township violence since the end of apartheid.

The attacks started in Alexandria township last week but have since spread across communities and the central business district in South Africa's biggest city, hitting townships in the east of the city on Monday.

They have targeted mostly Zimbabweans and Mozambicans, the biggest groups among immigrants who are accused by the poor of taking jobs and fuelling the high rate of violent crime.

Women have been raped, shops and homes looted and dozens of shacks burnt to the ground. Scores have been arrested.

"This is a war," said Lucas Zimila, a 60-year-old Mozambican man who was attacked by a machete-wielding mob while sleeping in his shack in Tembisa, north of Johannesburg, on Sunday night.

"They screamed at me to get out, that I didn't belong here. Then they burned everything in my house," said Zimila, who suffered a five-inch gash in his head.

The unrest is an embarrassment for South Africa, which has vaunted its tolerance since the end of apartheid and hopes to encourage foreign visitors for the soccer World Cup in 2010.

The violence is an indicator of anger among those who complain they have been left out by Mbeki's policies to promote business and investment. Investors are already worried by growing labour influence in the ruling ANC since Mbeki lost the leadership in December to rival Jacob Zuma.

ANC Treasurer General Matthews Phosa called at the weekend for an early election, calling for strong leadership. Mbeki has to step down next year and Zuma is the frontrunner to succeed.

Unease

South Africa, with a population of 50 million, is home to an estimated 5 million immigrants. Foreigners from poorer countries have been lured by work in mines, farms and homes and by one of the world's most liberal immigration and refugee policies.

"The violence is not a good signal for investors," said Raoul Luttik, senior investment manager, ING Investment Management in the Hague, Netherlands on a day the rand held steady.

"But the focus on the rand is more related to inflation, which has been spiralling, and also crucially, the monetary policy," he said.

Immigrants say that far from being criminals they are more often the victims of crime. Several told Reuters organised criminals were using the violence as cover to rob and loot.

Acting National Police Commissioner Tim Williams said in a statement that more officers would be deployed to trouble spots.

South African papers carried photos of a man set alight by a mob at the weekend. The scenes recalled troubles during apartheid, when activists fought security forces and rival factions. Suspected informants were sometimes burned to death.

Mbeki and Zuma have called for an end to the violence.

Anti-apartheid icon Nelson Mandela said he was saddened by rising hatred of foreigners. Archbishop Desmond Tutu, another Nobel laureate, pointed out that neighbouring states took in South Africans during the struggle against white minority rule.

"We can't repay them by killing their children," he said.

Hundreds of immigrants have taken refuge in police stations, churches and government offices.

"It's getting worse. They keep coming here because this is a safe place," said Simon Ramollo, a community activist who was arranging bedding and meals for about 200 foreigners.

The biggest group of immigrants -- an estimated 3 million -- are from Zimbabwe. They have fled economic collapse at home and the violent political standoff since disputed March 29 elections gives them little incentive to return home.

Mbeki's critics say his softly, softly approach to Zimbabwe has done too little to end the crisis there.