Ahmed Nejb Chebbi, who heads the National Salvation Front, Tunisia's main opposition coalition, was called in for questioning Friday by the counterterrorism squad as part of an investigation into claims of a "plot against state security."
"Today, as you can see, all forms of opposition, all independent opinion... is considered a crime that could lead to jail," Chebbi told reporters before being quizzed by investigators for three hours.
"In Kais Saied's Tunisia, the place of free men is in prison," Chebbi, 78, said.
Chebbi accused Saied on Friday of having "destroyed all institutions" and said the president was not aiming for "the rights and freedoms of citizens."
He also questioned the motive behind his interrogation saying "What am I being punished for? My right to think freely, to speak freely and to act within the framework of the law?".
Chebbi was free to leave after the investigation.
Earlier this month, human rights watchdog Amnesty International condemned plans to summon Chebbi for questioning "over unfounded accusations of conspiracy."
Police in Tunisia, birthplace of the 2011 Arab Spring uprisings, have arrested more than 20 government opponents since February, including former cabinet ministers, trade unionists and media figures.
One of the most prominent personalities detained is Rached Ghanouchi, the leader of the Islamist-inspired Ennahdha party who was sentenced in May to one year in prison on terrorism-related charges.
Before his detention in April, Ghannouchi, an 81-year-old former speaker of parliament, warned that efforts to stamp out political Islam and the left might lead to "civil war."