Along with millions of Brazilians, Bolsonaro and Lula also headed to the polls.

RIO DE JANEIRO — Among the first Brazilians to vote on Sunday were President Jair Bolsonaro and his leftist challenger, Luiz Inácio Lula da Silva.

Mr. Bolsonaro went to cast his ballot as soon as the gates opened at his polling location in a bucolic neighborhood on the western side of Rio de Janeiro, which is home to many military officers. Mr. Bolsonaro, a former Army captain, used to live in the neighborhood and has continued to vote there for years.

He wore a bright yellow shirt that said “Brasil,” in line with the patriotic clothing that has become a symbol of his movement. His supporters wear the yellow and green of Brazil’s flag, as well as the jerseys of Brazil’s national soccer team, to show their support for the president.

Beneath his shirt, Mr. Bolsonaro appeared to wear a protective vest, which he has been wearing at campaign events in recent weeks. Weeks before the first round of voting in 2018, Mr. Bolsonaro was stabbed in the stomach at a crowded rally, and he has continued to experience related health problems since.

“Our expectation is a victory for the good of Brazil,” he told reporters in brief comments, surrounded by security guards. “We only have good news, and we will be victorious.”

Roughly an hour later, Mr. da Silva, known universally as Lula, voted in a working-class city outside São Paulo, where he rose to become a union leader decades ago, starting his political career.

In a white button-down shirt, he posed for photographs after voting, flashing an “L” with his fingers — a symbol of his campaign — and kissing the slip of paper that voters receive after casting their ballots.

“This country, unfortunately, has gone backwards,” he told journalists. “And now we need to push it forward again. But not only for a part of society. We have to do it as a whole.”