Dilma Rousseff, elected as the first female Brazilian president four years ago, is leading in opinion polls, while another female politician, former environment minister Marina Silva, is a strong challenger.
As none of the three front-runner candidates is likely to garner more than 50 percent of the votes, a run-off is expected to be held on Oct. 26.
Silva, presidential candidate of the Brazilian Socialist Party, joined the race in the middle, beginning as the running mate of Eduardo Campos, who died in a plane crash in August. From a distant third place, Silva rose quickly and passed Aecio Neves of the Brazilian Social Democracy Party in the opinion polls to become runner-up.
According to the latest polls released Thursday evening, Neves is technically tied with Silva and has a chance to enter the run-off.
Brazilians will also choose state governors.
In Sao Paulo state, incumbent Geraldo Alckmin has taken the lead, trying for a fourth term as governor. He may win in the first round of elections, as polls show he has nearly 50 percent of support.
In the state of Rio de Janeiro, incumbent Luiz Fernando Pezao has the lead. Runner-up Anthony Garotinho, a former Rio governor, and third-ranked Senator Marcelo Crivella are fighting to face him in the second round.
Campaigning on radio and television ended Thursday evening, but candidates are allowed to campaign in the streets, without big rallies, until Saturday.
As Brazil has electronic ballots all over the country, the results of the first round of elections are expected to be known Sunday evening.