Flávio Bolsonaro, the far-right frontrunner to become Brazil's next president, has vowed to "re-democratise" the South American country, increasing fears about a move into authoritarian rule if he wins power.

Brazil was re-democratised in the 1980s after a brutal 21-year dictatorship that Flávio's father, the disgraced ex-president Jair Bolsonaro, has long praised.

Last year, Jair Bolsonaro, a former soldier who served under that military regime, was jailed for 27 years for trying to engineer a coup after he lost the 2022 election to the current leftwing president, Luiz Inácio Lula da Silva.

Flávio, 45, beat Lula, 80, in Sunday's presidential election first round and goes into the runoff on 25 October as favourite. On Tuesday, the rightwinger vowed to capitalise on the hugely increased far-right presence in congress after the election to change the constitution and "re-democratise this country".

"The only thing standing in the way of Brazil becoming a barbarity is called Flávio Bolsonaro as president," he claimed at a campaign event in Goiânia, a city near the capital, Brasília.

The incendiary comments caused outrage among historians and opponents of Bolsonaro's radical populist movement, who have accused him of co-opting the language of democracy to disguise an illiberal shift.

Talíria Petrone, the leftwing congresswoman, wrote on X: "This is the announcement of a coup."

Marcelo Freixo, a newly elected congressman from Lula's Workers' party (PT), said: "Brazil was re-democratised in 1988. The person who tried to undo this was his father, who was convicted of an attempted coup. What they want, once again, is a coup."

Bolsonaro did not spell out what he meant by "re-democratising" Brazil, a vibrant if polarised multiparty democracy that the Sweden-based V-Dem institute ranks 28th on its liberal democracy index, two places ahead of the UK and 23 ahead of the US.

But Jair Bolsonaro, who has praised authoritarian leaders including Chile's former dictator Augusto Pinochet, has long hankered for a return to military rule.

Last September, the supreme court found Jair Bolsonaro guilty of trying to convince military chiefs to help him cling to power at the end of his 2018-22 presidency. The conspiracy culminated in the 8 January 2023 riots in Brasília, a far-right insurrection seemingly inspired by the 6 January 2021 storming of the US Capitol.

Writing in O Globo newspaper, Míriam Leitão, a prominent political and economic journalist, warned that if Bolsonaro won, Brazil faced "the extreme and concrete danger of … another coup d'état attempt".

"Flávio follows the same ideas as his father and has already said that an intervention in the institutions is necessary," Leitão wrote. "If Flávio Bolsonaro is elected, he will have more tools at his disposal to carry out his father's authoritarian project: robust caucuses in the lower house and senate, a weakened supreme court, and the support of the Donald Trump administration.

"Political science shows that it's in the second term that an autocrat doubles down on their plan. Look at the case of Donald Trump. The risk isn't of the son repeating the father, it's that he surpasses him."

Miguel Lago, a political scientist, said Sunday's far-right election triumph – in which Bolsonaro's Liberal party (PL) secured the senate's largest caucus, with 28 of its 81 seats – would permit radical changes, including the impeachment of supreme court ministers considered enemies of the Bolsonarismo movement.

Jair Bolsonaro and his jailed co-conspirators are likely to be freed if his son wins, with the former president expected to become an influential minister able to select the new heads of the armed forces.

"It's better than a coup d'état … it's almost like a coup d'état with the support of the majority of the population," Lago said.

Nayib Bukele, El Salvador's authoritarian president, has been accused of "killing" democracy by using control over the country's congress and institutions to approve sweeping constitutional changes, including scrapping presidential term limits so he can seek indefinite re-election.

Flávio Bolsonaro recently met Bukele's security minister, Gustavo Villatoro, in São Paulo, hailing the Central American country's hardline security policies under which thousands have been jailed during a state of exception that has lasted nearly five years.

Bolsonaro told reporters that during their conversation, Villatoro "stressed the importance" of using control of congress to make the "necessary legal changes".

Celso Rocha de Barros, a political scientist, said he believed that by using the word "re-democratise", Flávio Bolsonaro sought "to smuggle his authoritarian purposes into acceptable language".

Barros said Bolsonaro, unlike his father who always seemed to favour an "old school barracks uprising" to seize power, appeared to be part of "a new generation of authoritarians" who sought to dismantle democracy from within by undermining and occupying the institutions.

"Then you don't need to break any laws to achieve authoritarianism … because you are the law," Barros added, citing Hungary's former far-right prime minister Viktor Orbán as an example. "Clearly, Flávio's plan is a corrosion of democracy so [Brazil] becomes a Hungary or Bukele-style system."

Danilo Araújo Marques, a historian, said the word "democracy" had been co-opted before in Brazil by undemocratic forces, when the military men and politicians behind the 1964 coup called their actions "an intervention to democratise the country".

"They all talked about democracy … [but] democracy has a different meaning in the mouth of a coup-monger," said Marques, who is from a research unit at the Federal University of Minas Gerais called Projeto República.

Marques noted that during the campaign, Bolsonaro had claimed his octogenarian opponent, Lula, represented the past while he was selling the future. "But this future means going back to what [Brazil] was before," Marques said. "It's a 'future' that is based on the order prior to the 1988 constitution, which is precisely the military dictatorship."

Asked to clarify his plans on Wednesday, Bolsonaro rejected speculation he was seeking to finish "what his father didn't do". "Let's stop lying. Let's touch down in the real world," he told reporters, adding: "Brazil needs to go back to normal, that's all."