One of the closest allies of Brazil’s ex-president Jair Bolsonaro – the former defence minister Gen Walter Braga Netto – has been arrested by federal police in connection with an alleged plot to seize power through a military coup.

Last month police formally accused Braga Netto, who was Bolsonaro’s vice-presidential running mate in the 2022 election, of playing a lead role in a criminal conspiracy designed to stop the vote’s leftwing winner, Luiz Inácio Lula da Silva, taking office. Another 39 people, including Bolsonaro, have also been accused.

Braga Netto, 67, has rejected those claims as “fanciful and absurd”.

But on Saturday morning, the four-star general turned politician, who also served as Bolsonaro’s chief of staff during his 2019-2023 presidency, was taken into custody at his home in Rio’s beachside Copacabana neighbourhood.

In a statement, the federal police said Saturday’s arrest was intended to prevent “the repetition of illegal acts”.

“During its investigations, the federal police concluded that Braga Netto was the architect of the coup [plot] – that he was the main authority behind the planning of the coup,” the political reporter Andréia Sadi wrote on her blog. “He was, in the words of one investigator, ‘the head, the mentor of the coup’ – but under the command of Bolsonaro.”

Bolsonaro, who remains at liberty, has also denied being part of a coup plot. “This is not true – I always acted within the limits of the constitution,” he told a rightwing conference in Buenos Aires, Argentina, last week.

Political observers now see the populist’s arrest as almost inevitable and speculate it will happen next year.

The recently published police report on the alleged plot paints a shocking picture of a long-running conspiracy investigators believe was designed to help Bolsonaro cling to power despite losing the 2022 election. The idea was allegedly to use social media to disseminate false claims of electoral fraud that could be used to justify a military intervention.

Police claim the pro-Bolsonaro plot was supposed to take place on 15 December 2022 – two weeks before Lula was due to be sworn in after beating Bolsonaro in October’s presidential election.

According to police, conspirators, including Braga Netto and other top military figures, allegedly hoped that on that day Bolsonaro would sign a “coup decree” authorising a military takeover. The next day, Braga Netto and the former minister of institutional security Gen Augusto Heleno were supposed to be put in charge of a “crisis management” cabinet.

Police claim the only reason Bolsonaro did not sign that decree was because plotters failed to secure sufficient support from Brazil’s military top brass.

Police also suspect the plotters of planning to assassinate or kidnap perceived enemies of Bolsonaro’s radical political movement, including Lula, his centre-right vice-president, Geraldo Alckmin, and the supreme court judge Alexandre de Moraes.

Earlier this month Bolsonaro’s former aide-de-camp Lt Col Mauro Cid reportedly told investigators that Braga Netto had helped bankroll the group of rogue special forces operatives that was supposedly behind those assassination and kidnap plans. Braga Netto delivered those funds disguised as a box of wine, according to the news outlet ICL Notícias.