President Lula (C) has thrown his support behind Dilma Rousseff (L)
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.
The latest polls published on Saturday suggested Ms Rousseff’s attempt to win enough votes to avoid a run-off vote on 31 October would be extremely tight.
O Globo newspaper’s prediction had Ms Rousseff winning 51% of the vote, with Mr Serra on 31%; the Folha de Sao Paulo newspaper poll put Ms Rousseff on 50% and Mr Serra on 31%.
Polls have consistently suggested Ms Rousseff would win a second round by a wide margin, but analysts say her position would be strengthened if she can win outright on Sunday.
Brazil, 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.”
Brazil: Key FactsEconomy: Set for some 7.5% growth this yearResources: Top exporter of sugar, poultry and beef; major producer of iron oreEnvironment: Amazon rainforest makes Brazil a key presence in climate talksInternational voice: Growing ties with Africa and Middle East; supporter of G20 roleSport: Hosts football World Cup in 2014 and Olympic Games in 2016
Ms Rousseff, 62, served as Mr Lula’s chief of staff from 2005 until this year, 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.
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