Ben Mbarek, co-founder of the National Salvation Front opposition alliance who is on hunger strike to protest his detention, was tried over his rebuke of legislative elections in 2022 which he had called a "ridiculous coup d'etat."
Held over "conspiracy against state security," Ben Mbarek remains behind bars as he awaits proceedings in other cases.
Tunisian and international rights groups have described the article under which Ben Mbarek was sentenced as a form of "repression" of free expression in the North African country, where President Kais Saied has ruled by decree since a 2021 power grab.
"Jawhar Ben Mbarek was sentenced without having the option to defend himself" due to his health condition after 13 days without food, his lawyer Ayachi Hammami told AFP.
Along with other figures detained since February 2023 over national security charges, Ben Mbarek has been on hunger strike to protest what he called the "arbitrary and unfounded" grounds for his arrest.
Ben Mbarek "was unable, due to the state of his health, to... be present at the hearing," the lawyer said, adding that the judge had proclaimed the verdict despite a request to postpone the proceedings.
Ayachi said the defense plans to appeal the ruling.
Ahmed Nejib Chebbi, another co-founder of the National Salvation Front, told AFP that Ben Mbarek "was sentenced to six months in prison for an opinion. I am truly outraged."
Rached Ghannouchi, the jailed leader of the Islamist-inspired Ennahdha party which is part of the opposition alliance, suspended his hunger strike on Wednesday for fear of health complications.
Rights groups have repeatedly condemned the proceedings against Tunisian opposition figures and called for their release.
Elected democratically in October 2019 with a five-year mandate, Saied launched a power grab in July 2021, dismissing the prime minister and suspending parliament.
He later pushed through sweeping changes to concentrate power in his office, shaking the foundations of the only democracy born out of the first Arab Spring uprisings in 2011.
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