"What is important are people's voices and clean elections, without any problems. May the best one win," he told journalists but avoided answering questions about whether he would respect the election's outcome.
Most opinion polls have shown Lula with a solid lead for months, but Bolsonaro has signaled he may refuse to accept defeat, stoking fears of institutional crisis or post-election violence. A message projected on Rio de Janeiro's Christ the Redeemer statue ahead of the vote read: "Peace in the Elections."
Voting in São Bernardo do Campo, Lula acknowledged the dramatic turnaround in his fortunes after a conviction that he says was politically motivated.
"It's an important day for me," he said. "Four years ago, I couldn't vote because I was the victim of a lie ... (I want) to try to help my country to return to normal."