Brazilians are preparing to elect the successor to President Luiz Inacio Lula da Silva, who is stepping down after two terms in office.
Mr Lula's favoured successor, his former chief of staff Dilma Rousseff, is widely expected to win the election.
But she needs 50% of the vote to avoid a run-off poll later this month.
Analysts say Ms Rousseff has run a careful campaign, benefiting from Mr Lula's widespread popularity and the country's booming economy.
Opposition candidate Jose Serra, of the Social Democratic Party, has pinned his hopes on getting enough votes to force a second round.
But opinion polls suggest Ms Rousseff would win a second round by a wide margin.
Armed struggleBrazil, one of the world's most-populous democracies, is also choosing local and national representatives in the election.
Most polling stations open at 0800 (1100 GMT), with some in the far east opening an hour earlier, and others in the west an hour later.
Polling stations use machines to log the votes, and preliminary results are expected within hours of the stations closing at 1700 local time.
Ms Rousseff, of the ruling Workers' Party, saw her lead in the opinion polls slip in the final days of campaigning after corruption allegations surfaced involving a former aide.
But her campaign has been boosted by energetic support from Mr Lula, who is constitutionally barred from standing for a third consecutive term.
"I'm convinced the majority of people want continuity from the government," Mr Lula told a rally on Friday.
"That's why I think Dilma will win".
Ms Rousseff, 62, has served as Mr Lula's chief of staff since 2005 and is a career civil servant. Her tilt at the presidency is her first attempt at elected office.
During the 1960s and 1970s she was involved in the armed struggle against Brazil's military rulers, and was jailed for three years.
The 68-year-old Mr Serra is hugely experienced, having served as Sao Paulo mayor, Sao Paulo state governor and health minister under President Fernando Henrique Cardoso. He lost the presidential election in a run-off to Mr Lula in 2002.
Two other candidates for the presidency are trailing far behind in the polls.
Marina Silva of the Green Party and Plinio de Arruda Sampaio of the Socialism and Freedom Party are not expected to trouble the frontrunners.
BBC News
