The son of former president Jair Bolsonaro, Flavio Bolsonaro has refused to concede defeat in Sunday's presidential election, saying he would seek the annulment of Lula's victory after the Supreme Court extended voting by two hours.
Later on Monday, however, Bolsonaro's Liberal Party said it acknowledged the run-off result in a statement signed by the party's head, Valdemar Costa Neto.
"We recognise the presidential election results," Costa Neto said.
"But we believe the issues raised need to be properly clarified."
Bolsonaro said earlier on Monday he would comply with a federal police order to hand over his passports following a visit to his father, who is under house arrest, and denied he was planning to flee Brazil.
Trump did not endorse Bolsonaro before the vote, although Reuters has reported that officials from his State Department led a campaign to return the Bolsonaros to power.
"I didn't take sides, but it looked like another rigged election," Trump told reporters, without providing evidence.
"It looked very bad to me."
Bolsonaro lost to Lula by more than four million votes, a clear result that it will be challenging for him to overturn.
Fewer than 35,000 votes, or some 0.03 per cent of the total, were cast in the extra two hours of voting ordered by the Supreme Court, according to an analysis of electoral records by Insper professor Pedro Burgos.
"I just think it had a very bad look, the election, the way it was tabulated," Trump said.
"The extension given was very strange."
Vice-President Geraldo Alckmin later defended Brazil's electoral system, saying in a TV interview with Rede Vanguarda that Trump's remarks were "totally wrong" and "unfounded".
Neither Trump nor Jair Bolsonaro accepted their election losses in 2020 and 2022, respectively.
Brazil's former president was handed a 27-year sentence for plotting a coup after losing to Lula.
Analysts do not expect Flavio Bolsonaro to be able to build the same anti-democratic movement as his father in 2022.
World leaders have already widely congratulated Lula on his unprecedented fourth mandate.
Lula's victory defied polls, precedent and global anti-incumbent sentiment to put the brakes on a wave of right-wing election victories across Latin America.
He trailed Bolsonaro in the first round, but managed to convince voters to come out for him in the run-off.
During the campaign, Lula pointed to his record of cutting poverty and inequality while also highlighting record-low unemployment, illiteracy and deforestation.
But his fourth term might prove challenging, with his agenda facing an uphill battle in a Congress packed with Bolsonaro allies.
