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Brazilian President Luiz Inacio Lula da Silva turned 81 on Tuesday, promising to work "24 hours a day" following his historic re-election for a fourth term.
Lula won Sunday's runoff against Flavio Bolsonaro with 51.8 percent of the vote to the right-wing senator's 48.2 percent.
He is one of the world's oldest leaders. US President Donald Trump, who turned 80 in June, is only a few months younger.
"I'm going to deliver the best term any president has ever served in the history of this country," the leftist leader said in a video posted on Instagram.
"I will dedicate 24 hours a day to making this country into the country everyone dreams of," he added.
Amid questions over his age, he boasted while campaigning that he was "fitter than ever", jogging during events and posting videos on social media of his gym workouts.
Following his shock first-round defeat to Flavio Bolsonaro, the son of jailed ex-leader Jair Bolsonaro, Lula's campaign had a social media makeover.
The president bought his first-ever cellphone and declared himself "king of the selfie."
In a speech after his dramatic victory on Sunday, he thanked the "rebellious youth" of Brazil for campaigning on his behalf.
Lula's Workers' Party (PT) called on supporters to celebrate his birthday outside the presidential residence in Brasilia on Tuesday.
Bolsonaro passports cancelled , whereabouts unclear
Brazil's Flavio Bolsonaro is politically isolated after refusing to accept election defeat, had his passports cancelled on Tuesday as speculation swirled over his whereabouts.
Bolsonaro, 45, was defeated by incumbent Luiz Inacio Lula da Silva on Sunday and is set to lose his parliamentary immunity as his legal woes pile up.
While he cried foul and vowed to challenge the election legally, his party and allies have accepted the result, leaving him increasingly isolated.
Bolsonaro last spoke publically on Monday afternoon in a video on social media saying he had been banned from leaving the country and ordered to surrender his passports.
However, while he said he would comply with the order, he had failed to do so by Tuesday afternoon.
"Flavio Bolsonaro's passports have been cancelled," a police source told AFP on condition of anonymity, adding that "there is no formal record of Flavio Bolsonaro leaving the country."
Argentina's security ministry and migration authorities on Tuesday denied local media reports that Bolsonaro had entered the neighboring country.
Bolsonaro had said the order to surrender his passports was based on a comment by his US-based brother Eduardo Bolsonaro, who told a US podcast two days before the election that "if my brother does not win, I am in favor that he would leave the country."
Flavio Bolsonaro is under legal scrutiny in several cases, including a corruption probe over the financing of the biopic "Dark Horse" about his father, former president Jair Bolsonaro, who is under house arrest for plotting a coup.
"I never had any intention of leaving the country. I will surrender my passports and defend myself against this travesty," Flavio Bolsonaro said.
Foreign interference probe
Members of Flavio Bolsonaro's inner circle did not respond to AFP requests for comment Tuesday on his movements.
AFP observed a motorcade leaving his residence in Brasilia, though it could not be determined whether the senator was inside any of the vehicles.
Lula, who turned 81 on Tuesday, staged a dramatic comeback after losing to Bolsonaro in a first-round vote.
After a revamped campaign and massive mobilisation from the left, he pulled around 4.4 million more votes than his rival.
Bolsonaro refused to accept the result, complaining about an extension of voting hours and accusing Lula of an abuse of powers over a flurry of social spending announcements during the campaign.
While congratulations rolled in for Lula from world leaders on both sides of the political divide, US President Donald Trump, an ally of the Bolsonaro family, said the election "looked terrible to me."
"I didn't take sides but it looked like another rigged election frankly," he told reporters in Washington. "It looked so corrupt."
After the bitter election race, a Supreme Court justice on Tuesday ordered police to investigate alleged foreign funding of Bolsonaro's election campaign, and other interference from abroad such as social media "manipulation."
Justice Flavio Dino ordered the investigation of "the possible origin, circulation and destination of foreign resources" during the campaign.
He also ordered the probe of "deliberate asymmetries and algorithmic manipulation by internet companies aimed at favoring or harming candidacies in Brazil's electoral process."