More than 142 million Brazilian voters are due to go to the polls to choose the country’s next president, following a dramatic election campaign.
Early on, incumbent Dilma Rousseff had been expected to win outright.
However, the death of one of the main candidates and the popularity of his replacement, environmentalist Marina Silva, means the vote is now likely go to a second round.
Ms Rousseff’s other main rival is Aecio Neves, a centrist, pro-business figure,Drive Hot News reports.
Voting starts at 0800 (11:00 GMT) and voters will also select a federal parliament and state governors.
Mr. Neves was narrowly ahead of Ms Silva, according to several opinion polls published on Saturday, which all put Ms Rousseff in a commanding lead with about 45 per cent.
Brazil’s voting system is all electronic and is likely to deliver a result within an hour or two of polls closing at 22:00GMT.
On the eve of the vote, the Superior Electoral Court moved to dismiss reports that the system was vulnerable to hacking, saying it was “fraud-proof,” despite frequent attempts to hack into the ballots.
During Ms Rousseff’s tenure as president, unemployment has been lower than under any of her predecessors, at about five per cent. The minimum wage has risen and the number of undernourished Brazilians has been falling.
But the past 18 months have been marred by recession and corruption scandals, and protests over poor public services and the World Cup costs