President Kais Saied sacked the government and froze parliament in a 2021 power grab and imposed sweeping changes to the political system.

More than 20 political figures have been arrested in the North African country in recent weeks, including members the main opposition coalition, the National Salvation Front (NSF) and its main component, the Islamist-leaning Ennahdha party.

"Freedom for the detainees," chanted the demonstrators, largely supporters of the NSF, with many waving Tunisian flags and pictures of detainees, AFP journalists said.

Denouncing Saied's power grab as a "coup", the protesters defied a ban on demonstrations imposed by Tunis authorities.

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 ask 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 heads the NSF.

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.

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 Sunday's demonstration, which came a day after more than 3,000 joined a Tunis rally organised by the powerful UGTT trade union.

During that rally, UGTT chief Noureddine Taboubi accused Saied of targeting the union as part of a wider crackdown against critics, and called on him to accept "dialogue".