There had been little doubt the "Yes" campaign would win, and an exit poll suggested that votes cast -- just a quarter of the 9.3 million electorate -- were overwhelmingly in favor
"Tunisia has entered a new phase," Saied declared as he addressed celebrating supporters in downtown Tunis hours after polling stations closed.
"What the Tunisian people did... is a lesson to the world, and a lesson to history on a scale that the lessons of history are measured on," he said.
A poll of "Yes" voters by state television suggested "reforming the country and improving the situation" along with "support for Kais Saied/his project" were their main motivations for backing the constitution.
Thirteen percent cited being "convinced by the new constitution".
There had been little doubt the "Yes" campaign would win, and an exit poll suggested that votes cast -- just a quarter of the 9.3 million electorate -- were overwhelmingly in favor.
The turnout, projected at around 22 percent, was "quite good" given about two million people have been automatically added to electoral rolls since the 2019 legislative elections, he told AFP.
Most of Saied's rivals called for a boycott, and while turnout was low, it was higher than the single figures many observers had expected -- at least 27.5 percent according to the electoral board, controlled by Saied.
The National Salvation Front, a coalition of Saied's main opponents, said the draft constitution would enshrine in a "coup d'etat" and that "75 percent of Tunisians have refused to approve a putschist project."
Rights groups and legal experts have warned that the draft gives vast, unchecked powers to the presidency, allow him to appoint a government without parliamentary approval and make him virtually impossible to remove from office.
The charter "gives the president almost all powers and dismantles any check on his rule," Said Benarbia, regional director of the International Commission of Jurists told AFP.
"None of the safeguards that could protect Tunisians from Ben Ali-type violations are there anymore," he added.
Analyst Abdellatif Hannachi said the results meant Saied "can now do whatever he wants without taking anyone else into account."
"The question now is: what is the future of opposition parties and organizations?"
As well as remaking the political system, Monday's vote was seen as a gauge of Saied's personal popularity, almost three years since the political outsider won a landslide in Tunisia's first democratic direct presidential election.
Monday's referendum came a year to the day after Saied sacked the government and suspended parliament in a dramatic blow to the only democracy to have emerged from the 2011 Arab Spring uprisings.