Police raided Ennahda party headquarters early on Tuesday and evacuated all present to start a search that will take days, after showing a judicial warrant, party officials said.
"A police unit showed up at the party's main headquarters (in Tunis) and ordered everyone there to leave before closing it," Riadh Chaibi said.
"The police also closed the other offices of the party elsewhere in the country and prohibited any meeting in these premises," he told AFP.
The move came after Ennahdha's veteran leader Ghannouchi was arrested at his home in the capital Tunis, the latest in a string of opposition figures held.
Ennahdha vice-president Mondher Lounissi told a news conference late Monday that Ghannouchi had been taken to a police barracks for questioning and that his lawyers had not been allowed to attend.
His arrest came after media reports in which he allegedly said Tunisia would be threatened with "civil war" if political Islam, from which his party originated, were eradicated in the country.
A source at the interior ministry quoted by Tunisian media confirmed that Ghannouchi's arrest was linked to these statements.
The party leader's daughter, Yusra Ghannouchi, said her father's detention deprived him of a basic right.
"They are still refusing to allow lawyers to attend the interrogation," she said, adding "they are insisting that he can be held for 48 hours without access to a lawyer. In the meantime, they are insisting to persist with the interrogation, but my father is refusing to speak without the presence of lawyers as is his legal right."
"They insisted on keeping him sitting in the interrogation room all night," she stated.
She also made it clear that Tunisia must have more than one voice, stating "Anyone imagining Tunisia without this or that group, a Tunisia without Ennadha, without political Islam, without the left or any of its components, would be laying the ground for civil war."
Along with the Ennahdha leader's arrest, other party members cuffed are Mohamed Goumani, Belkacem Hassan and Mohammed Chnaiba. Officials in Ennahda confirmed the arrests.
Ghannouchi's lawyers said that they had no idea about the course of investigations.
Since Saied's seizure of broad powers, which the president says was needed to save Tunisia from years of crisis, Ghannouchi has been the biggest political party leader to oppose him.
Police have this year detained leading political figures who accuse Saied of what they call a coup.
These arrests, which have led to charges of conspiring against state security, have drawn statements of concern from the U.S. and rights groups.
William Lawrence, professor at American University in Washington, D.C., said he doubts Tunis can keep up the heavy fist, commenting that "The latest moves by Kais Saied show a lack of political skill and a lack of geopolitical skill which are sure to worsen Tunisians' political, and economic security."
