Rached Ghannouchi, leader of Tunisia's biggest and most organised political party Ennahda, arrived for questioning by an investigating judge at an anti-terror centre as his supporters rallied outside the courthouse in the capital Tunis.
Main opposition party of the north African country urges supporters to protest on Friday against President Kais Saied.
Tunisia's national prevention of torture body said authorities have not provided any information on the arrest of former deputy president of the Ennahda party.
Accusing the Tunisian President Kais Saied of staging a coup for suspending the parliament, the party says the arrest signals a slide towards tyranny.
One member of the Ennahda party has died and several others have been injured after a fire erupted at the party's headquarters.
"The wrong political choices of the leadership of the Ennahda movement led to its isolation and failure," 113 senior officials from the party said in a joint statement.
Kais Saied also extended the suspension of the immunity of members of parliament, as the moves have raised concerns among Tunisians about the future of the democratic system in the country.
By closing political avenues for voters to express their views, the President Kais Saied risks extreme polarisation, and with it, the potential for unrest.
Imperfect and riven with political deadlock, Tunisia's democratic experiment was seen as a model, albeit imperfect. The country's elected president, however, sees the country's future under a different political system.
Supporters of Ennahda Party stage a demonstration on Saturday in the capital Tunis to strengthen national unity and for giving a message of the necessity of dialogue.
Outlets including Sky News Arabia and Al Arabiya published false stories about the leader and founder of the Ennahda party.
Amid a sharp spike in attacks and desecrations of mosques in Tunisia, some see a larger strategy at work trying to incite religious conflict and undermine the current government.
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