Ennahdha called seizure of Noureddine Bhairi a dangerous precedent that foreshadows a slide towards tyranny.
In a new decree, President Saied granted himself additional powers to rule over a reformed Supreme Judiciary Council.
The Ennahdha party said Ridha Bouziane, who was one of its members, was subjected to severe violence.
Critics fear President Kais Saied is growing increasingly authoritarian with move to bring judiciary under his control.
Opposition politicians and activists say President Kais Saieds monopoly of power is untenable.
Il presidente tunisino accusa l?organo giudiziario di corruzione e parzialità. La manifestazione per commemorare Belaid
The controversial decision is Kais Saieds latest since sacking the government last year.
The Supreme Judicial Council vows to continue working after the president orders its dissolution.
Tunis sees heavy police presence as hundreds rally against President Kais Saied despite COVID restrictions.
Janny Sikazwe, who officiated at the 2018 World Cup, blew for the final whistle on minute 85 and then on 89.