Brazil’s Supreme Court has sentenced former president Jair Bolsonaro to 27 years and three months in prison after convicting him of attempting to overturn the result of the country’s 2022 election, in a pivotal ruling for Latin America’s largest democracy.

Four out of five justices on a special panel of the court found the 70-year-old guilty on September 11 of leading what prosecutors called a criminal conspiracy to keep himself in power. He was convicted on five counts: plotting a coup d’état, attempting to abolish the democratic order by force, participation in an armed criminal organisation, aggravated damage to public property, and deterioration of protected heritage sites.

Justice Alexandre de Moraes, rapporteur in the case and a long-standing adversary of the former president, said Bolsonaro sought to “annihilate the essential pillars of the democratic rule-of-law state” and reimpose “dictatorship in Brazil”. Justice Cármen Lúcia, whose vote ensured the decisive majority, said there was “conclusive evidence” the defendant had acted “with the purpose of eroding democracy and institutions”.

The case arose from events surrounding Bolsonaro’s defeat to Luiz Inácio Lula da Silva in October 2022. Prosecutors alleged the coup plot began a year earlier with systematic efforts to undermine trust in Brazil’s electronic voting system. The scheme culminated on January 8 2023, when thousands of Bolsonaro supporters stormed and vandalised Congress, the Supreme Court and the presidential palace in Brasília, days after Lula’s inauguration.

Only Justice Luiz Fux dissented, arguing the January 8 assault was the work of a “disorderly mob” and not the outcome of an organised conspiracy. Bolsonaro, who has been under house arrest since earlier this year, denies all wrongdoing and retains the right to appeal to the full 11-member bench of the Supreme Court.

The unprecedented ruling has already triggered political and diplomatic repercussions. The sentencing marks the first time a former Brazilian head of state has been jailed for attempting to subvert the democratic order, underscoring the fragility of institutions in a country that emerged from military dictatorship less than four decades ago.

Tensions have spilled over into relations with Washington, where President Donald Trump, a close ally of Bolsonaro, denounced the proceedings as a “witch hunt”. “I saw the trial, I know it very well. As a foreign leader, I thought he was a good president. It’s very surprising that this could happen,” Trump said. He added: “It’s very similar to what they tried to do to me, but they didn’t get away with it.”

The Trump administration last month imposed 50% tariffs on most Brazilian exports in retaliation for the case, sanctioned Justice Moraes for “serious human rights violations”, and announced visa restrictions on other court officials. US secretary of state Marco Rubio has warned that Washington would escalate its response following the far-right leader’s sentencing.

Brasília, though, has vowed not to bow to external pressure. The foreign ministry said “threats like the one made today by US Secretary of State Marco Rubio … will not intimidate our democracy”. President Lula has long described Trump’s interference as “unacceptable” and “an assault on Brazil’s sovereignty”. Speaking hours before the verdict was confirmed, he told local broadcaster Band that Brazil had no fear of fresh US sanctions, according to Reuters.

The conviction of Bolsonaro, still a polarising figure with a fervent support base, is likely to deepen domestic tensions. His defence team is expected to challenge the ruling, setting the stage for further clashes in a country still reckoning with its most serious institutional crisis since the end of military rule.