Accusing the Tunisian President Kais Saied of staging a coup for suspending the parliament, the party says the arrest signals a slide towards tyranny.
Tunisian security forces have detained a senior official from the biggest party in the suspended parliament for the first time since President Kais Saied seized governing powers in July.
The Ennahda party, which accuses Saied of a coup for freezing the parliament and accumulating powers, said plainclothes agents seized Noureddine Bhairi on Friday morning and took him away.
It called the arrest a dangerous precedent that may foreshadow a slide towards tyranny.
Saied has promised to uphold rights and freedoms won in Tunisia's 2011 revolution that ushered in democracy and triggered the Arab Spring uprisings across the region.
However, he has brushed aside the democratic 2014 constitution and given himself powers to rule by decree during a transitional period in which he will offer a new constitution to public referendum.
Ennahda, which has the largest number of seats in the suspended parliament, was banned before the revolution but then became the most consistently influential party afterwards and a member of successive coalition governments.
However, as Tunisia's economy stagnated and its political system ground to paralysis in recent years, support for the party waned and although it came first in the 2019 parliamentary elections it won far fewer votes than in previous years.