Brazil’s Supreme Court on Tuesday began weighing the case against Jair Bolsonaro, the former president charged with planning a coup after losing the 2022 election, kicking off a trial years in the making that will pose a major test of the nation’s democracy.

But Mr. Bolsonaro himself was not there.

He didn’t appear in court because of poor health, his defense team told reporters outside the courthouse. Ahead of the trial, one of his sons told a Brazilian news site Mr. Bolsonaro had been suffering from debilitating hiccups that the former president attributes to complications from a stabbing attack on the campaign trail in 2018.

Mr. Bolsonaro, 70, has been under house arrest, wearing an ankle monitor, for weeks. Then, over the weekend, Brazilian authorities tightened security measures, placing police officers around his home amid worries that he may try to flee and possibly seek refuge at the U.S. Embassy near his home.

Prosecutors accuse Mr. Bolsonaro and seven members of his inner circle of overseeing a vast conspiracy to cling to power after his defeat at the polls in 2022, seeking to overturn the results, trying to enlist the military in their plot and drafting plans to poison Mr. Bolsonaro’s rival, Luiz Inácio Lula da Silva, who is now Brazil’s president.

Mr. Bolsonaro and the other defendants face criminal charges, including the “violent abolition of the democratic rule of law” and “coup d’état.” If found guilty, Mr. Bolsonaro could be sentenced to more than 40 years in prison.

Mr. Bolsonaro denies he plotted a coup or planned to kill Mr. Lula but, in pretrial testimony in June, he told the court that he had “studied ways within the Constitution” to remain in power after his electoral defeat. Most analysts, pointing to a trove of evidence, expect a majority of the five-member Supreme Court panel to convict Mr. Bolsonaro.