Brazilians will vote on Sunday to choose a new president to succeed Luiz Inacio Lula da Silva.
Opinion polls suggest the governing Workers Party candidate Dilma Rousseff has a clear lead over Jose Serra of the opposition Social Democratic Party.
Ms Rousseff has enjoyed the full support of President Lula, who is leaving office after two terms with record popularity ratings.
If she wins she will become Brazil's first woman president.
Polls open at 1000GMT and close at 2000GMT.
Brazil uses an electronic voting system, and final results are expected within hours of the polls closing.
This second round of voting was forced after Ms Rousseff fell short of the 50% needed in the 3 October first round. She won 47% to Mr Serra's 33%.