Luiz Inacio Lula da Silva, the Brazilian leftist leader, narrowly defeated President Jair Bolsonaro in a runoff election on Sunday night, but the far-right incumbent did not concede defeat, raising fears that he would contest the outcome.

Lula was declared the next president by the Supreme Electoral Court (TSE) with 50.9% of the vote to Bolsonaro's 49.1%. The inauguration of Lula, 77, is scheduled for January 1.

It was a stunning comeback for the leftist former president and a crushing defeat for Bolsonaro, Brazil's first presidential incumbent to lose.

"So far, Bolsonaro has not called me to acknowledge my victory, and I'm not sure if he will call or recognize my victory," Lula told tens of thousands of ecstatic supporters on Sao Paulo's Paulista Ave.

The president will not make public remarks until Monday, according to a source close to the Bolsonaro campaign. The Bolsonaro campaign did not respond to a request for comment.

Bolsonaro openly discussed not accepting the election results last year, claiming that Brazil's electronic voting system was vulnerable to fraud.

In an apparent response to the results, one close Bolsonaro ally, lawmaker Carla Zambelli, tweeted, "I PROMISE you, I will be the greatest opposition that Lula has ever imagined."

Investors are gauging speculation about Lula's cabinet and the risk of Bolsonaro questioning results, which could lead to a volatile week in financial markets.

The vote was a rebuke to Bolsonaro's fiery far-right populism, which emerged from the back benches of Congress to forge a novel conservative coalition but lost support as Brazil suffered one of the worst coronavirus pandemic death tolls.

US President Joe Biden congratulated Lula on his victory in "free, fair, and credible elections," joining a chorus of praise from European and Latin American leaders.

International election observers said the election on Sunday was well-run. According to one observer, military auditors found no flaws in the voting system's integrity tests.

According to the highway operator, truck drivers believed to be Bolsonaro supporters blocked a highway in four places on Sunday in the state of Mato Grosso, a major grains producer.

In one online video, a man stated that truckers planned to block the country's main highways, calling for a military coup to prevent Lula from taking office.