Air force helicopters were deployed to back up South African police in Johannesburg's Alexandra township after President Thabo Mbeki called for army intervention to help end unrest that has threatened to destabilise Africa's largest economy.

A top leader of the ruling ANC criticised police for reacting too slowly to 11 days of attacks on African migrants, which have driven at least 15,000 people from their homes and prompted thousands to return to their countries.

"10,047 returned home in buses provided by the government, the number is likely to increase in the next days as long as violence unfolds in South Africa," Mozambique's Deputy Immigration Director Leonardo Boby told Reuters in Maputo.

The armed mobs accuse the African immigrants of stealing jobs and fuelling crime. Several people have been burned to death and scores of shacks looted and torched. Most of the immigrants are from Zimbabwe and Mozambique.

The deputy leader of the ruling African National Congress, which ousted Mbeki as party leader in December, criticised the police delay in responding to the violence which erupted in Alexandra township on May 11 and spread rapidly.

"The delay encouraged people in similar environments to wage similar attacks against people who came from our sister countries on the continent," Kgalema Motlanthe said at an international media industry conference in Johannesburg.

"We are confronted by one of the ugliest incidents in the post-apartheid era," said.

Motlanthe said the violence was an assault on the values of South Africa's democratic society. He is a close ally of ANC leader Jacob Zuma, who defeated Mbeki for the party leadership.

INSTABILITY

The attacks on African migrants have increased political instability at a time of power shortages and disaffection over Mbeki's pro-business policies. Soaring food and fuel prices helped push tensions to breaking point.

The South African currency fell sharply earlier this week on the back of the violence. The rand was slightly firmer on Thursday at 7.7235 to the U.S. dollar.

The biggest group of immigrants come from Zimbabwe. An estimated three million have fled economic collapse at home.

Zimbabwe's opposition leader Morgan Tsvangirai visited Alexandra on Thursday and met with Zimbabweans sheltering at the police station there.

"We do understand the problems you are facing and in fact we are shocked by the plight of men and women who have left their countries," Tsvangirai said, adding that the violence was linked to his country's political crisis.

One refugee said he was considering going home despite the fact that in Zimbabwe he would have to face hyperinflation, shortages of food and an upsurge of political violence since disputed March 29 elections.

"I am thinking of going back to Zimbabwe, I'm too scared," said Samuel Dhliwayo, a 30-year-old Zimbabwean who worked as a painter.

South Africa has a population of about 50 million and is home to an estimated 5 million immigrants.

Its reputation as a haven for immigrants and asylum seekers is in tatters, and there are growing fears that the crisis could dent the country's lucrative tourism industry and cripple its hosting of the 2010 soccer World Cup.