"We call for a boycott of the referendum because what is being voted on is not in the interests of Tunisians," the party's spokesman Imed Khemeri said in Tunis on Thursday.
"The draft that has been presented has been made to measure" for Saied, Khemeri said, adding "This document did not come from the people or from a national dialogue."
Around 30 NGOs including the SNJT journalists' union and rights group the LTDH also called Thursday for a boycott of the referendum, saying the text was "written by a single person without participation by civil society or experts".
The legal expert who headed a committee to draw up the new charter said the final text published by Saied had "nothing to do with the text we drafted and submitted to the president".
The expert, Sadeq Belaid, added that it risked paving the way for a "dictatorial regime", more than a decade after Tunisia's pro-democracy revolt sparked copy-cat uprisings across the region.
Ennahdha, which has dominated Tunisia's politics since the revolt that toppled dictator Zine El Abidine Ben Ali in 2011, has found itself under pressure since Saied's power grab.
A Tunisian court on Tuesday froze the bank accounts of its chief and speaker of the now-dissolved parliament Rached Ghannouchi, as well as his son Mouadh and former prime minister and one-time senior party member Hamadi Jebali, who is accused of money-laundering.
Saied, in an open letter on Tuesday, defended the proposed constitution.
He said "this draft was built on what the Tunisian people have expressed from the start of the revolution up until the correction of its path" last July, and asserted that those who worried about it creating a new autocracy hadn't read it properly.
