Protesters burning tyres and barricading roads in townships around South Africa's capital have highlighted divisions in the ruling African National Congress.

Those factions are threatening the party's grip on Pretoria and cities including the commercial hub, Johannesburg, in August 4 municipal elections.

The protests erupted on Tuesday after the ANC's national executive committee said Kgosientso Ramokgopa, the Mayor of the Tshwane municipality that includes Pretoria, wouldn't be its candidate in the election.

Instead, the party nominated Thoko Didiza, the chairwoman of the National Council of Provinces, Parliament's second house.

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"This is really bad for the ANC because over and above the factional issues that have been there for quite some time, now there's the problem in uniting behind a mayoral candidate," Dirk Kotze, a politics professor at the University of South Africa in Pretoria, said. "The NEC's focus was to try and find someone who can help build bridges between the different factions but I think it has had the opposite effect."

While the ANC is still credited with leading the fight against white-minority rule and has won more than 60 per cent support since taking power in 1994, mounting discontent over a lack of basic services and a series of scandals implicating President Jacob Zuma has boosted the opposition's campaigns.

A June 7-8 survey of 3000 potential voters in Tshwane, Johannesburg and Nelson Mandela Bay by research company Ipsos for broadcaster eNCA showed the ANC is set to lose control of all three municipalities. The Democratic Alliance topped the rankings in Tshwane, with 33 per cent support, while the ANC polled 28 per cent and the Economic Freedom Fighters 10 per cent.

"Tshwane is divided, down the middle,'' Nkenke Kekana, a spokesman for the ANC in the central Gauteng province, said by phone. "If we are going to start letting branches choose the mayors then there would be revolt. We don't know why the members choose to go out onto the streets and sought things out. It is the ANC that they vote into power not individuals. We are confident that this will pass."

Ramokgopa, the ANC's deputy chairman in Tshwane who has been at loggerheads over recent months with his deputy Mapiti Matsena, told Johannesburg-based PowerFM on Tuesday that he supports Didiza's nomination as mayor.

"It is always the case that a small number of people are able to generate quite a lot of turmoil," Sandy Africa, a politics professor at the University of Pretoria, said. "There is no real indication of the extent of the split. Whether this is just a small faction or whether it's a widespread phenomenon, only a more scientific count would be able to tell us."

The 278 municipalities oversee parks, libraries, sanitation, some roads and distribution of electricity and water, and get most of their funding from real-estate taxes and transfers from the national government. The ANC currently controls seven of the country's eight biggest cities, while the DA runs Cape Town.

The Ipsos poll showed the ANC had 31 per cent support in Johannesburg, the DA 29 per cent and the EFF 10 per cent. In Nelson Mandela Bay, which includes the city of Port Elizabeth, the DA had 34 per cent backing, the ANC 30 percent and the EFF 7 per cent. Between 17 and 21 per cent of respondents in each of the cities said they were undecided about who they would vote for. The DA and EFF have said they are prepared to enter into coalitions with other opposition parties but not the ANC.

- Bloomberg