Hundreds of Tunisians rallied in defiance against an official ban on Sunday's protest, to demand the release of more than 20 prominent figures opposed to the president.
They include members of the main opposition coalition, the National Salvation Front (NSF), and its main component, the Islamist-leaning Ennahdha party, who were arrested recently.
They denounced a 2021 power grab by President Kais Saied 'as a coup' after he sacked the government and froze parliament. He also imposed sweeping changes to the political system of the sole democracy to have emerged from the Arab Spring uprisings.
However the president says the move was essential to save Tunisia from chaos.
Initially dozens gathered by a key bus and tramway station in central Tunis before charging police barricades to then march towards Habib Bourguiba Avenue, where the crowd soon swelled to more than 500, reporters said.
A policeman used a loudspeaker to urge demonstrators to move out of the iconic avenue -- the site of repeated protests -- and head towards the headquarters of Al Joumhouri party several kilometres away, saying: "Please, the march is banned".
Issam Chebbi, head of Al Joumhouri party, is among the Saied opponents who have been arrested in the crackdown launched in February.
His brother Ahmed Nejib Chebbi, who heads the NSF, addressed the crowd and said the arrests were "arbitrary".
Protester Ahmed Nejib Chebbi, 78, said: "We are defending a national cause and we will not stop until democracy and institutions return."
Other detainees include senior opposition figures Jawhar Ben Mbarek, businessman Kamel Eltaief, the head of Tunisia's most popular radio station Mosaique FM, Noureddine Boutar, as well as trade union officials.
Mbarek's father, Ezzedine Hazgui, who was imprisoned under the dictatorship of ousted president Zine El Abidine Ben Ali, attended the rally and denounced Saied in comments to AFP.
"The president has placed all state institutions under his control and has divided the people. The police protects an illegitimate president," Hazgui said.
Saied has accused those arrested of "terrorism" and causing recurrent food shortages, as well as plotting against the state.
Rights group Amnesty International has labelled the arrests a "politically motivated witch hunt".
The NSF had called for the demonstration, which came a day after more than 3,000 joined a Tunis rally organised by the powerful UGTT trade union.