The vote is seen as the toughest test yet for the governing African National Congress (ANC) party, 25 years after it swept to power at the end of white minority rule.
South Africa's ANC surged into the lead in early official results on Thursday with 55 percent of the vote in the first electoral test of President Cyril Ramaphosa's bid to reinvigorate the ruling party.
With just over a fifth of voting districts tallied, the Election Commission put the African National Congress (ANC) well ahead, with its closest rival the Democratic Alliance (DA) trailing with a distant 26 percent.
The Economic Freedom Fighters, founded six years ago by former ANC youth leader Julius Malema, was sitting at eight percent.
Ramaphosa, 66, took over last year after the ANC forced then-president Jacob Zuma to resign after nine years dominated by corruption allegations and economic problems.
The party that wins the most seats in parliament selects the country's president, who will be sworn in on May 25.
"The outcome of this election will be a major boost for investors... and investor confidence, it's about confidence and about the future," Ramaphosa said after voting on Wednesday.
The ANC's reputation was badly sullied under Zuma.
TRT World's Ben Said has more.
Support for the ANC has fallen in every election since 2004 with the party taking 54 percent in 2016 municipal elections, compared with 62 percent in 2014's national vote.
Nelson Mandela and the ANC were swept to power with a landslide in the country's first multi-racial polls that marked the end of apartheid in 1994.
Most opinion surveys suggest the ANC will secure nearly 60 percent of the vote, thanks to Ramaphosa's appeal and a fractured opposition.
"The higher the percentage for the ANC, the more it will give him (Ramaphosa) bargaining power," Dirk Coetzee, a professor at University of South Africa's political science department, said.
"If Ramaphosa gets below 50 percent he will be very vulnerable" to challenges from rivals within the ANC, he added.
'The poor are getting poorer'
The ANC has been confronted by deepening public anger over its failure to tackle poverty and inequality in post-apartheid South Africa.
"We have given them 25 years but the poor are getting poorer and the rich richer," said voter Anmareth Preece, 28, a teacher from Coligny in North West province.
"We need a government that governs for the people, not for themselves," Preece said.
The economy grew just 0.8 percent in 2018 and unemployment hovers around 27 percent; soaring to over 50 percent among young people.
Of the 47 opposition parties in the race, only the main opposition centrist DA and the radical-left EFF are major players.
The DA is hoping to shed its image as a white, middle-class party.
Its first black leader, Mmusi Maimane, is contesting his maiden general election since taking the helm in 2015 and is expected to make modest gains on the DA's 2014 vote share of 22 percent.
Malema's EFF was predicted to make major gains, growing from 6.3 percent to a forecast 11 percent. The partial results show it at eight percent.
"The ANC has taken people for granted. There is some arrogance which has crept in," said voter Mandla Booi, 45, in Port Elizabeth on the south coast.
The EFF, which appeals mainly to young voters and the poor, has campaigned on a policy of seizing land from white owners to give to blacks.
Enforced land redistribution is also an ANC policy – alarming some investors.
About 26.8 million voters were registered to cast their ballots at 22,925 polling stations countrywide.