The NEC was meeting to discuss whether to remove Mbeki, who is mired in accusations that he conspired to undermine ANC leader Jacob Zuma.
Tension had been mounting all day as the NEC delayed a media announcement on the matter, which had been expected by 5.30pm.
By 7.30pm, ANC secretary general Gwede Mantashe briefly addressed the media gathered outside the conference centre and said: "We are not finished. We are in the middle of the debate."
When journalists pressed him on any news of the Mbeki matter, he retorted: "You want me to speculate?"
ANC spokesperson Jessie Duarte said that an announcement would only be made by Saturday morning.
Mantashe also said the NEC was "not discussing Thabo Mbeki. It is discussing the Judge [Chris] Nicholson judgement in its entirety. At the end of that discussion, we will come and brief you."
The plan is to replace Mbeki with parliamentary Speaker and ANC chairperson Baleka Mbete, but this will not be done through a parliamentary vote of no-confidence or by impeachment.
Instead, all stops will be pulled out to get Mbeki to step down voluntarily and avoid a public battle. Mbeki indicated to his Cabinet this week that he will not defy the ANC’s leadership if they tell him to go.
It is understood that the NEC’s core leadership, the national working committee, decided at a meeting on Monday to recommend Mbeki’s removal as president.
After the NEC’s decision, the intention is to send a high-ranking delegation to ask Mbeki to do "the honourable thing" by stepping down and avoiding the embarrassment of being ousted by a motion of no confidence.
Removing Mbeki from the Presidency could rattle investors with whom he is popular because of his pro-business policies. But he has been on thin ice with powerful trade unions and other Zuma supporters, who accuse Mbeki of trying to undermine the man who replaced him as ANC leader.
Fired as Mbeki’s deputy president in 2005 after he was linked to alleged wrongdoing in the multimillion-rand arms deal, Zuma defeated Mbeki in a bitter leadership contest late last year that ended at the ANC’s Polokwane conference in December. Zuma is seen as the front-runner to succeed him as head of state next year.
Mbeki is barred by the Constitution from a third term as state president.
The move to oust the president picked up speed after Judge Chris Nicholson last week in the Pietermaritzburg High Court ruled that corruption charges against Zuma were not legal, and said there had been high-level meddling in the case.
Zuma’s camp has branded the prosecution a political witch-hunt by Mbeki and his aides.
Mbeki, who has consistently denied he hatched a political conspiracy against Zuma, lashed out at his critics on Friday even as the NEC — which is dominated by Zuma allies — met to discuss his fate.
"It impoverishes our society that some resort to the tactic of advancing allegations with no fact to support these," the president said in a statement issued by his office. He said he was not involved in the National Prosecuting Authority’s decision to appeal against Nicholson’s ruling.