Saied, a conservative, won the presidential election earlier this month. He succeeds the late Beji Caid Essebsi, who died in office in July.
Kais Saied, a political outsider and retired law professor, was sworn in as Tunisian president on Wednesday after he won a landslide victory in this month's election.
Saied's win delivered a heavy blow to a governing elite accused of failing to improve living standards or end corruption since the 2011 revolution that introduced democracy after years of authoritarian rule.
Even with a large mandate, the new president has less direct control of policy than the prime minister and both will quickly face a series of tough challenges, including high unemployment rates and fighting corruption.
"There will be no tolerance in wasting any millime [cent] of the money of our people," Saied said in a speech.
He vowed to protect freedoms, saying, "Those who have nostalgia to go back to the old years only pursue illusions and mirages," he added.
Tunisia’s president controls foreign and defence policy, governing alongside a prime minister chosen by parliament who has authority over domestic affairs.
But Saied said he envisaged a bigger state role in the economy.
"We are moving from frustration to construction and work," he added.
Tunisia has a deeply fragmented legislature in which the largest party, the conservative Ennahda, has only 52 of the 219 seats.
Saied wants Tunisians to elect small local councils based on the character of their representatives rather than party or ideology. They would, in turn, choose regional representatives who would choose national ones.