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South Africa's ruling African National Congress is marking 105 years since it was formed but there are deep divisions in the ranks of Africa's oldest liberation movement.

President Jacob Zuma, ANC leader, will speak to party members on Sunday in Soweto, Johannesburg.

The party, which has won every election since the end of apartheid in 1994, has been plagued by infighting, corruption allegations and public protests - most recently by university students.

It will pick a new leader at a conference in December and, given its national dominance since coming to power, the winner is likely to go on to be South Africa's next president when elections are held in 2019.

Nkosazana Dlamini-Zuma, the chairwoman of the African Union and a former wife of Jacob Zuma, is viewed as a frontrunner. She is a Zulu, the largest tribe in South Africa, and is expected to have the backing of her former husband, who will have a major say in who succeeds him.

Dlamini-Zuma was regarded as a capable technocrat during her time as South Africa's minister of home affairs between 2009 and 2012 and has since gained international exposure as the first female head of the AU.

However, critics of Dlamini-Zuma, a medical doctor trained in South Africa and Britain, say she should have done more to intervene when former president Thabo Mbeki denied that HIV causes AIDs and imposed anti-scientific policies.

Vice President Cyril Ramaphosa, a unionist-turned-business tycoon, is viewed as her most likely rival after powerful trade unions endorsed him last year.

Neither Dlamini-Zuma, 67, or Ramaphosa, 64, have declared their intention to run.

Ramaphosa, who was once touted as a successor to Nelson Mandela, would be the first choice for many investors because his background in commerce suggests he will support more pro-business policies than many in the traditionally left-wing ANC.

However, he will face criticism from opponents for his role at platinum producer Lonmin where he was a director and shareholder when violence led to police shooting dead 34 striking miners in 2012.

An investigation has cleared him of wrongdoing.