General elections are expected in the first half of next year.
The size of the crowd exceeded organisers' expectations and about one third of them were not able to fit into the convention hall.
Earlier Mosiuoa Lekota, the former defence minister, opened the convention with a traditional ANC chant "Amandla!" - or "Power!" - and led the crowd in cheering, "Forward with the convention, forward!"
"We have no apology to anybody. We have decided and we are ready to fight as messengers and representatives of hope for the people," he said.
Lekota said that the ANC is concentrated on interfering in state institutions and working to the benefit of its leaders.
The ANC has dominated South African politics since the end of apartheid in 1994.
But the bitter split created when the ANC forced Thabo Mbeki, the then president of South Africa, to resign before the end of his second term threatens to create an a viable alternative.
The meeting follows an attempt by the ANC to block the formation of the breakaway party.
|Lekota, former ANC chairman and Mbeki ally, leads the breakaway movement [AFP]
Late on Friday, a Pretoria high court deferred ruling on an application filed by the ANC "to prevent the use of names or designations that may be confusingly similar to the name and trademark of the ANC".
Lekota officially resigned from the party on Friday over its treatment of Mbeki. He had already been suspended by the ANC after he suggested he might form the new party.
"The last few weeks have been very intense in South Africa," Al Jazeera's Zeina Awad, reporting from Johannesburg, said.
"Insults have been directed at the leaders of this breakaway movement, they have been called 'dogs' and 'traitors' and on a couple of occasions their meetings were broken up.
"People are saying the ANC has betrayed its democratic principles ... representatives from across the country are coming together to try and hammer out a new platform. The ANC has said they are taking this quite seriously.
"What [the breakaway faction] are calling for is more consultation, more democracy and the inclusion of voices other than the ones who are currently in government," she said.
"We've seen members of opposition groups, we've seen civil society here today - South Africans from all walks of life coming together in these consultations."
Mbeki has distanced himself from Lekota's move, but in letter published in The Star newspaper on Friday he refused to endorse the ANC and asked that his name not be used during its election campaign.
Mbeki stepped down in September after a judge suggested he may have interfered in the prosecution of Jacob Zuma, his successor as ANC president and the man widely tipped to be South Africa's next president.
Mbeki strongly denied the claims.
The ANC won more than two-thirds of votes in the last election and controls a strong majority in parliament.