Polls are due to open in South Africa's fifth general election since the end of apartheid 20 years ago.
The governing African National Congress (ANC) is tipped to win, returning President Jacob Zuma for a second five-year term.
However, it is expected to lose ground amid concern over high unemployment and a number of corruption scandals.
The run-up to the vote has been marked by protests and troops have been deployed to boost security.
Wednesday's election is the first time that those born after the end of apartheid are able to take part and commentators say much will depend on how they cast their ballots.
Polls show many are disaffected with the country's leadership and are willing to support the opposition Democratic Alliance, led by anti-apartheid activist Helen Zille, or the Economic Freedom Fighters, headed by former ANC youth leader Julius Malema.
The ANC's campaign has drawn heavily on past glories and on the outpouring of grief over the death of its former leader, Nelson Mandela.
"Do it for Madiba, Vote ANC!" campaign posters read, referring to Mr Mandela by his clan name.
But many commentators say this election could be the last to be dominated by South Africa's post-apartheid legacy.
About a quarter of South Africa's workforce is jobless and a BBC poll suggests unemployment is the major issue among young voters, followed by education.
Some 22,000 polling stations will open at schools, places of worship, tribal authority sites and hospitals, while dozens of vehicles serving as mobile voting centres will operate in remote areas.
About 25 million people have registered to vote – roughly half the population.
Police say at least one officer will be on duty at every polling station and troops have also been deployed to keep order at various hotspots.
There was rioting in Bekkersdal township, south west of Johannesburg, on Tuesday and reports that some temporary polling stations had been burned down.
Bekkersdal has suffered intermittent unrest since last year as residents protested over a lack of public services. Many have vowed to boycott the election.
Polls are due to open at 07:00 (05:00 GMT) and close 14 hours later.
President Zuma is expected to vote at his Nkandala homestead in rural KwaZulu-Natal while opposition leader Helen Zille will vote in Cape Town.
The full result is not expected before Friday.