The normally frictionless presidential vote at the South Africa's ruling African National Congress was marked by bickering over how the votes would be counted and a bitter public rivalry.

President Thabo Mbeki and Jacob Zuma were formally nominated for party president on Monday.

The vote was originally scheduled for Sunday, but was stalled by disputes between supporters of the two men. Polling of roughly 4,000 delegates is scheduled to start Tuesday.

Zuma, a former guerilla fighter backed by the left, is a favourite to win and snatch the leadership from Mbeki. If he does, he is almost certain to become South Africa's president due to ANC's overwhelming support among the country's black majority.

Mbeki has to stand down from his post in 2009, according to the constitution. However, staying at the helm until then would give him a say in who succeeds him and what policies the next leader adopts.

The political squabble between the two men and their loyal delegates is the biggest rift the ANC has seen in recent years.

During the underground movement to fight apartheid in past decades, the ANC consistently presented a united front. The top party post hadn't been publicly contested in 55 years.

Party rift 'saddens' former president Nelson Mandela

Former president Nelson Mandela, 89, who has retired from public life but is still a respected figure, expressed his concern about the deep divide within the party.

"It saddens us to see and hear of the nature of the differences currently in the organization," he said in a message to delegates distributed by the South Africa Broadcasting Corporation.

The divide continued to play out publicly Monday as delegates cheered for their respective candidates during rallies at the conference.

Delegates loyal to Mbeki waved their hands in three-fingered salutes to symbolize their desire for a third term for the current president. Zuma supporters instead gestured for change by rolling one hand over another, as if to call for a substitute during a soccer game.

Mbeki has been more pointedly rejected during the party congress, with delegates calling for him to change his aloof manner and criticizing, what some say, is his failure to provide housing, employment and services 13 years after the end of apartheid.

Zuma has made a remarkable political comeback after a rape trial in 2006. It ended with his acquittal, but left lingering questions about his judgment. He outraged AIDS activists by testifying that he had unprotected, consensual sex with the HIV-positive woman and then took a shower in the belief that it would protect him from the virus.

Zuma may also face a pending corruption inquiry. In 2005, Mbeki fired Zuma as the country's deputy president after Zuma's financial adviser was convicted of trying to elicit a $70,000 bribe for Zuma to deflect investigations into an arms deal.

Charges were withdrawn against Zuma but the National Prosecuting Authority has indicated it may revive them.

With files from Associated press