The poll by Sigma Conseil said 92.3% of voters in the referendum supported the new Constitution, which with no minimum participation rate is now set to become law. Turnout was 25%, the exit poll showed, Reuters reported.
Critics of Saied accuse him of a coup for ousting the elected parliament a year ago and moving to rule by decree – steps they say will be formalised in the new Constitution, dismantling the democracy won in a 2011 revolution.
The new Constitution gives the president power over both the government and judiciary while removing checks on his authority and weakening the parliament.
Saied has said his moves were needed to save Tunisia from years of political paralysis and economic stagnation under a 2014 Constitution that split power between the parliament and president.
His initial moves against the parliament appeared hugely popular with Tunisians, as thousands flooded the streets to support him, but with little progress in addressing dire economic problems, that support may have waned.
The lowest turnout of any national election since the 2011 revolution, which triggered the Arab Spring, was 41% in 2019 for the parliament that Saied has dissolved.
The president's opponents have also questioned the integrity of a vote conducted by an electoral commission whose board Saied replaced this year, and with fewer independent observers than for previous Tunisian polls.