President Thabo Mbeki called for unity in the ruling African National Congress as he faces losing the party leadership to his bitter rival at a crucial party conference. The ANC's deputy president, Jacob Zuma, whose reputation has survived rape charges but who is still under investigation for corruption, is likely to become South Africa's next president on the back of a rebellion within the ranks of the party that has seen the ANC presidency being contested for the first time in 55 years. In an attempt to present a unified front, Mbeki and Zuma arrived on the stage together on Sunday but barely spoke as they sat in front of the crowd of 4,000 delegates. The conference began on South Africa's National Reconciliation Day - but there was little unity reflected in the highly charged atmosphere. Defiant delegates booed those leaders regarded as Mbeki supporters and carried pictures of Zuma high above their heads as they repeatedly sang the anti-apartheid song "Bring me my machine gun," which has become Zuma's anthem. Delays in proceedings meant voting did not go ahead as scheduled Sunday but was now expected to happen Monday. Evoking the memory of former President Nelson Mandela, from whom he took over in 1999, Mbeki said corruption, abuse of power and bitter internal divisions were like a "disease" threatening to destroy the ANC. In a 2Â½-hour speech to the conference, Mbeki described "matters" affecting Zuma as being "one of the most difficult and painful challenges we have faced over the last five years." Mbeki fired Zuma as the country's deputy president in 2005 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 Charges were withdrawn against Zuma but the National Prosecuting Authority has indicated it may revive them. Zuma was last year acquitted of raping a family friend, but 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 AIDS virus. Zuma survived both those scandals, successfully portraying himself as a victim of a plot to stop him from becoming president. Mbeki on Sunday called for unity and a restoration of the "moral force of our movement." "During the years since our liberation in 1994, certain negative and completely unacceptable tendencies have emerged within our movement, which threaten the very survival of the ANC as the trusted servant of the people it has been for 96 years," he said. Mbeki is barred by the constitution from seeking a third term as president of Africa's political and economic powerhouse. But remaining at the helm of the ANC would give him a say in who succeeds him and in the policies his successor adopts. Much has been made of the personality and class differences between the two former allies, who are both 65 and spent years in exile during apartheid. Mbeki, a foreign-educated academic who sprinkles his speeches with Shakespeare, is seen as aloof. He spearheaded the country's economic boom but has alienated the poor, who feel they have not benefited nearly 13 years after the end of apartheid. Zuma, who had no formal schooling and was a leader of the exiled ANC's military wing, is much more populist and has strong backing from the union movement, which wants him to push through more pro-poor policies. "This is a serious divide around the nature of development in our society," South African political analyst Adam Habib said in an interview before the conference. "There is a rebellion in the ranks of the ANC." Before the ANC congress, Zuma called for AIDS and crime to be treated as national emergencies, something many South Africans have criticized Mbeki for not doing. He also criticized Mbeki for sticking to quiet diplomacy in the face of Zimbabwe's political repression and economic meltdown. Zuma held talks with business leaders both in South Africa and abroad to try to soothe investor concerns about the impact of his becoming ANC - and ultimately national - president. In his organizational report, the ANC secretary-general, Kgalema Motlanthe, criticized the "ferocious lobbying" by the two camps and said the party's leadership "has failed to resolve the divisive issues that have plagued the movement in the past few years." "If we are not vigilant we could easily slide down a dangerous path ... the process of preparing for this conference has revealed numerous problems that require urgent correction," continued Motlanthe. Meanwhile, two of South Africa's most powerful moral voices have spoken out on the matter. Mandela, 89, who has retired from politics but who is still seen as a unifying figure in the country, expressed his concern at the divisions in 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 the delegates distributed by the South Africa Broadcasting Corporation. "Whatever decision you are to make at this conference, including decisions about leadership positions in the organization, let the noble history of the ANC guide you." On Friday, Archbishop Desmond Tutu, one of the country's most powerful moral voices, urged the ANC not to elect Zuma, someone whom he came out against following the rape charge. Tutu pleaded with delegates to "not choose someone of whom most of us would be ashamed."