While the party of the late Nelson Mandela looked likely to remain the largest political force, voters appeared to have punished the former liberation movement for years of decline.
With results in from 57.3% of polling stations, the ANC had 41.9% of votes, a precipitous drop from the 57.5% it secured in the last national election in 2019.
The pro-business Democratic Alliance (DA) was in second place on 23.4%. uMkhonto we Sizwe (MK), a new party led by former president Jacob Zuma, was at 11.3% and eating into ANC support, particularly in KwaZulu-Natal, Zuma's home province.
MK had overtaken the Marxist Economic Freedom Fighters (EFF), currently the third biggest party in parliament, which was sitting on 9.5%.
"I think it's a very good day for South Africa. We said for the last 30 years the way to rescue South Africa was to break the ANC majority. We've done that," DA leader John Steenhuisen told reporters at the results centre in Midrand, north of Johannesburg.
The ANC had won every previous national election since the historic 1994 vote that ended white minority rule, but over the last decade South Africans have watched the economy stagnate, unemployment and poverty climb and infrastructure crumble, leading to regular power outages.
Speculation was intense about which party or parties the ANC may approach to form a coalition and remain in government, or what other negotiations might be going on behind closed doors.
Asked if his party was in coalition talks, Steenhuisen said "no".
"We've got to wait for the results to end before we can start with any major discussions but my first port of call is going to be with my Multi-Party Charter conference," he said, referring to an alliance of opposition parties formed before the election.
The uncertainty impacted the government bond market, with prices of the country's main internationally traded bonds down as much as 1.3 cents on the U.S. dollar. The falls were the third in a row and left the bonds at their lowest level in almost a month.
'DOOMSDAY COALITION'
Projections by South Africa's Council for Scientific and Industrial Research indicated the ANC would get 40.5% when full results are in.
The ruling party's performance looked particularly dire in KwaZulu-Natal, where MK was on 44.1% to just 18.8% for the ANC, according to partial results as of 0830 GMT.
By law the election commission has seven days to release full provisional results, but election officials have said they are planning for a Sunday announcement.
The results page on the electoral commission's website, which had been updating seamlessly since the start of the count, went blank for roughly two hours early on Friday due to a technical problem. The data reappeared shortly after 0700 GMT.
"The data in the data centre remains intact and the results have not been compromised ... Result processing continues unaffected," the commission said in a statement.
Political parties' share of the vote will determine the number of seats they get in the National Assembly, which then elects the next president.
That could still be the ANC's leader, incumbent President Cyril Ramaphosa. However, an embarrassing showing at the polls risks fuelling a leadership challenge.
Investors and the business community have voiced concern over the prospect of the ANC entering a coalition with the EFF, which is calling for the seizure of white-owned farms and the nationalisation of mines and banks, or with Zuma's MK which also talks about land confiscation.
Though the DA says it wants to oust the ruling party, Steenhuisen has not ruled out a partnership to block what he has called a "doomsday coalition" with the ANC bringing the EFF or MK into government.
