With 37% of the national vote counted, the former liberation movement had 66.88% compared with the total 66.35% it scored in the last election in 1999.
Political analysts said the ANC was probably set for a two-thirds majority in parliament, which would allow it to amend the constitution at will.
"Our prediction is the ANC should be at around 69%," said Hans Ittmann of the Centre for Scientific and Industrial Research, a group analysing the results, adding that many regions of traditional ANC support were still to report.
But the ANC was battling for two key provinces and faced a rising challenge from the opposition Democratic Alliance (DA), which ran a campaign highlighting South Africa's huge problems with AIDS, crime, unemployment and poverty.
"Our prediction is the ANC should be at around 69%"
Centre for Scientific and Industrial Research
The DA, a largely white party which has sought to cement its position as the official opposition, almost doubled its support from the 1999 elections with a total of 16.54% of the national vote, according to early results.
The main national loser appeared to be the New National Party, heir to the grouping that built apartheid, which was at just 2.29% against almost seven percent in 1999.
"We are excited about the results so far," ANC spokesman Smuts Ngonyama said. "The hurricane is beginning to blow. It's blowing in our favour."
The ANC, which under Nelson Mandela swept away apartheid in 1994, appeared strong across the country with results putting it ahead in all nine provinces - albeit in some cases by small margins. Early national turnout estimates were about 74%.
Voter turnout was high
This was below the huge 89% in the last poll in 1999 but showed a continuing enthusiasm for multiracial democracy.
Wednesday's vote appeared smooth despite scattered accusations of intimidation and fraud in traditional hotspots and tension in the battleground province of KwaZulu-Natal.
The Zulu-based Inkatha Freedom Party (IFP) launched its first challenge to Wednesday's vote in the eastern province, a move analysts said could raise tensions further in a region which has a long history of political bloodshed.
"It may well be that we would not accept the election results if we believe electoral fraud and cheating have been sufficient to undermine the entire process," said IFP national spokesman Musa Zondi.
A heavy security presence kept a lid on political tensions in KwaZulu-Natal, where the IFP is fighting to keep the only provincial government under its control.
Opposition DA leader Tony Leon(L)
and IFP's Mangosuthu Buthelezi
Officials were forced to dispatch some 240 soldiers to the Zulu heartland to rescue 30 ANC agents after they were threatened by IFP supporters during the vote count.
Voters elected regional assemblies as well as the national parliament, and the ANC wants to capture both KwaZulu-Natal and the Western Cape - the only two of the nine provinces it does not already directly control.
In both cases, early results hinted that coalition governments could take office.
KwaZulu-Natal lagged the rest of the country in reporting results, but the race appeared extremely tight. With 22% of the polling stations reporting, the ANC was ahead with almost 42% of the vote against 41% for the IFP.
The Western Cape, with Cape Town,
remains a hotly contested province
The DA, which is in an electoral alliance with the IFP, scored 8.96% - a trend which, if sustained, could see the IFP-DA in control of the province.
In Western Cape, with more than half of polling stations reporting, the ANC was at 39.47% against 32.72% for the DA, which had hoped for a provincial victory.
The New National Party, which continues to draw some support from Cape Town's mixed-race population and now runs the province in coalition with the ANC, was polling at 11.8%, making another coalition government likely.