Deputy President Cyril Ramaphosa and former African Union Commission chair, Nkosazana Dlamini Zuma are vying for the top job with 4 776 voting delegates thought to be split down the middle.
Ramaphosa received 1 469 nominations from six provinces for president, while Dlamini Zuma received 1 094 nominations.
Delegates are also casting their ballots for the party's deputy president, national chairperson, secretary general, deputy secretary general, and treasurer general.
ANC spokesperson and National Executive Committee (NEC) Zizi Kodwa on Monday took to Twitter to reveal that he'd thrown his weight behind Ramaphosa.
"Am casting my vote for comrade Ramaphosa for ANC President, whatever happens I trust he will restore confidence and inspire hope, it's early hours but it's worth waiting," he tweeted.
Am casting my vote for comrade Ramaphosa for ANC President, whatever happens I trust he will restore confidence and inspire hope, it's early hours but it's worth waiting #ANC54
Police Minister Fikile Mbalula meanwhile, shared to his 1 million followers that he had voted for "mother princess" Dlamini Zuma.
ANC chief whip Jackson Mthembu, who has previously stated his support for Ramaphosa, also reiterated his stance.
Delegates also seemed to follow this trend, with some supporters from KwaZulu-Natal revealing that they had voted for Dlamini Zuma.
Two delegates expressed confidence that she would emerge victorious because of her support from the big provinces.
"She's supported by provinces with large delegations... Ramaphosa has many provinces but with smaller delegates," one delegate said.
Results are expected to be announced during the course of the day.
ANC spokesperson Khusela Sangoni has since said members had a right to demonstrate their preferences.
IOL and African News Agency/ANA