South Africa's ruling party has captured control of the "rebel" city of Cape Town after a court ruling gave municipal councillors a 15-day window to defect to other parties.

That gives President Thabo Mbeki's African National Congress (ANC) a clean sweep of all major cities.

In Cape Town, the ANC will govern in coalition with an unlikely partner - the New National Party (NNP), made up mainly of Afrikaners and Coloureds (mixed-race) - which ruled under apartheid.

The coalition displaces the mainly white Democratic Alliance (DA), which saw more than 300 of its councillors throughout the country defect to the NNP during the "window", which closed at midnight on Tuesday, various party officials said.

The government is now planning to introduce a technical amendment to the constitution to allow defections at the provincial and national level as well.

That could change the balance of power in the volatile eastern province of KwaZulu-Natal, currently controlled by the Zulu-dominated Inkatha Freedom Party (IFP).

IFP leader Mangosuthu Buthelezi, the national home affairs minister, reminded MPs in May, of bloody fighting between ANC and IFP supporters between 1985 and 1995 which left 12 000 dead, and warned that the situation in KwaZulu-Natal could become critical.

"People must realise that simmering underneath . . . we have not managed, in fact, to destroy all these fires that destroyed so many of our people," he told the national upper house, the National Council of Provinces.

The DA and the NNP earlier ruled both Cape Town and its Western Cape Province in coalition, but the two parties fell out late last year and the NNP jumped into bed with the ANC.

"From the time they broke away, the NNP attempted to find a new home and the ANC attempted to capitalise on the discord in the opposition," said Clive Napier, a political science lecturer at the University of South Africa in Pretoria.

"They (the NNP) were a party in steady decline. The main driving force was . . . to find a place for themselves in South Africa's political world."

A political researcher who did not want to be named said she agreed with Napier's view.

"It's such a small opposition . . . what they are trying to do is scramble for seats, and position themselves the best they possibly can for at least holding on to pockets of power, because they'll never get the national power.

"For them it's about survival, that's all this is," she said.

DA leader Tony Leon described the NNP's decision to co-operate with the ANC as betrayal, saying the NNP had "doomed itself to extinction" by turning its back on its own supporters.

"The people of South Africa are not stupid. They will not forget those who betrayed them and those who stuck with them," Leon said.

NNP leader Marthinus van Schalkwyk hit back saying opposition to the ANC-led government could never come from the DA.

"Opposition to the right of the ANC is dead," he said. "The need now is to strengthen the centre."

The executive head of South African political research institute IDASA, Paul Graham, said he believed the NNP had dealt the DA a body-blow.

"For the moment, who's in charge of the local government is what counts.

"The NNP managed to draw sufficient representation to control the large majority of local governments previously owned by the DA. - AFP