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 per cent of the vote to the right-wing senator’s 48.2pc.

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.”

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.