Brazilian leftist leader and former President Luiz Inacio Lula da Silva narrowly defeated the incumbent Jair Bolsonaro in a runoff election late Sunday.
However, far-right Bolsonaro was yet to concede defeat, raising concerns that he might contest the result.
The Supreme Electoral Court (TSE) declared Lula the next president, with 50.9% of the votes against 49.1% for Bolsonaro. The 77-year-old Lula's inauguration is scheduled for Jan. 1.
It was a stunning comeback for the leftist former president and a punishing blow to Bolsonaro, the first Brazilian incumbent to lose a presidential election.
"So far, Bolsonaro has not called me to recognize my victory, and I don't know if he will call or if he will recognize my victory," Lula told tens of thousands of jubilant supporters celebrating his win on Sao Paulo's Paulista Ave.
A source in the Bolsonaro campaign told Reuters the president would not make public remarks until Monday. The Bolsonaro campaign did not respond to a request for comment.
Bolsonaro last year openly discussed refusing to accept the results of the vote, making baseless claims that Brazil's electronic voting system was vulnerable to fraud.
One close Bolsonaro ally, lawmaker Carla Zambelli, in an apparent nod to the results, wrote on Twitter, "I PROMISE you, I will be the greatest opposition that Lula has ever imagined."
Financial markets might be in for a volatile week, with investors gauging speculation about Lula's cabinet and the risk of Bolsonaro questioning results.
The vote was a rebuke for the fiery far-right populism of Bolsonaro, who emerged from the back benches of Congress to forge a novel conservative coalition but lost support as Brazil ran up one of the worst death tolls of the coronavirus pandemic.
U.S. President Joe Biden congratulated Lula for winning "free, fair and credible elections," joining a chorus of compliments from European and Latin American leaders.
International election observers said Sunday's election was conducted efficiently. One observer told Reuters that military auditors did not find any flaws in the integrity tests they did of the voting system.
Truck drivers believed to be Bolsonaro supporters on Sunday blocked a highway in four places in the state of Mato Grosso, a major grains producer, according to the highway operator.
In one video circulating online, a man said truckers planned to block the country's main highways, calling for a military coup to prevent Lula from taking office.
Lula's win consolidates a new "pink tide" in Latin America, after landmark leftist victories in Colombia and Chile's elections, echoing a regional political shift two decades ago that introduced Lula to the world stage.
He has vowed a return to state-driven economic growth and social policies that helped lift millions out of poverty during two terms as president from 2003 to 2010. He also promises to combat the destruction of the Amazon rainforest, now at a 15-year high, and make Brazil a leader in global climate talks.
"These were four years of hatred, of negation of science," Ana Valeria Doria, 60, a doctor in Rio de Janeiro who celebrated with a drink. "It won't be easy for Lula to manage the division in this country. But for now, it's pure happiness."
A former union leader born into poverty, Lula organized strikes against Brazil's military government in the 1970s. His two-term presidency was marked by a commodity-driven economic boom and he left office with record popularity.
However, his Workers Party was later tarred by a deep recession and a record-breaking corruption scandal that jailed him for 19 months on bribery convictions, which were overturned by the Supreme Court last year.
