Former President Luiz Inácio Lula da Silva of the leftist Workers’ Party got the most votes in Brazil’s presidential election Sunday, but not enough to avoid a runoff vote against his right-wing rival, incumbent Jair Bolsonaro.

With 97 percent of the votes tallied, da Silva had 47.9 percent support and Bolsonaro 43.6 percent. Since neither candidate received more than 50 percent of the valid votes, a second round vote between them will be scheduled for October 30.

Brazil’s election authority announced late Sunday that a second round was a mathematical certainty.

The highly polarized election will determine whether the country returns a leftist to the helm of the world’s fourth-largest democracy or keeps the right-wing leader in office for another four years.

Mr. Bolsonaro’s administration has been marked by incendiary speech, his testing of democratic institutions, and his widely criticized handling of the COVID-19 pandemic.

He built a devoted base by defending conservative values, however, rebuffing political correctness and presenting himself as protecting the nation from leftist policies that he says infringe on personal liberties and produce economic turmoil.

In the run-up to the election, Mr. Bolsonaro repeatedly questioned the reliability not just of opinion polls, but also of Brazil’s electronic voting machines.

At one point, Mr. Bolsonaro claimed to possess evidence of fraud, but never presented any, even after the electoral authority set a deadline to do so. He said as recently as September 18 that if he doesn’t win in the first round, something must be “abnormal.”

Mr. da Silva, 76, was once a metalworker who rose from poverty to the presidency and is credited with building an extensive social welfare program during his 2003-2010 tenure that helped lift tens of millions into the middle class.

But he is also remembered for his administration’s involvement in vast corruption scandals that entangled politicians and business executives.

Mr. da Silva’s own convictions for corruption and money laundering led to 19 months imprisonment, sidelining him from the 2018 presidential race that polls indicated he had been leading against Mr. Bolsonaro. The Supreme Court later annulled his convictions on grounds that the judge was biased and colluded with prosecutors.

Speaking after casting his ballot in Sao Bernardo do Campo, the manufacturing hub in Sao Paulo state where he was a union leader, Mr. da Silva recalled that four years ago he was imprisoned and unable to vote.

“I want to try to make the country return to normality, try to make this country again take care of its people,” he told reporters.

Mr. Bolsonaro grew up in a lower-middle-class family before joining the army. He turned to politics after being forced out of the military for openly pushing to raise servicemen’s pay. During his seven terms as a fringe lawmaker in the Brazilian Congress’ lower house, he regularly expressed nostalgia for the country’s two-decade military dictatorship.

On Saturday, Mr. Bolsonaro shared social media posts by right-leaning foreign politicians, including President Trump, who called on Brazilians to vote for him. Israel’s former Prime Minister, Benjamin Netanyahu, expressed gratitude for stronger bilateral relations, and Hungarian Prime Minister Viktor Orbán also praised him.

After voting Sunday morning, Bolsonaro told journalists that “clean elections must be respected” and that the first round would be decisive. Asked if he would respect results, he gave a thumbs up and walked away.