Kais Saied, a political outsider and retired law professor, was sworn in as Tunisian president on Wednesday after a landslide win in this month's election.
Mr Saied's victory 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 to the country.
The conservative academic who had no political experience won the overwhelming support of younger voters in an October 13 runoff.
Mr Saied was sworn in before members of the Constituent Assembly and other top state bodies.
The poll followed the death in July of Beji Caid Essebsi, Tunisia's first president freely elected by universal suffrage.
Mr Saied has won a clear mandate to fight corruption and promote social justice, even though his role focuses on security and diplomacy.
A constitutional law professor whose rigid and austere demeanour earned him the nickname "RoboCop", he has no real experience in foreign policy.
Tunis, which current chair of the Arab League, could renew diplomatic ties with Syria, which were severed since 2012, and play a role in the return of the war-torn country to the bloc.
Mr Saied has made strong statements against Israel, considering any ties with the country to be "high treason", earning him praise among supporters.
While the security situation has significantly improved since attacks on tourists in 2015, Tunisia has maintained a state of emergency for four years, with assaults against security forces persisting.
On June 27, a suicide attack killed two people in the heart of the capital Tunis, reviving the spectre of violence.
During the campaign debate, Mr Saied said a key to fighting terrorism was education and that improving primary education would "immunise" youths against extremism.
Another significant task he will face is reforming the police force, which was a cog in the dictatorship toppled by the uprising of 2011 and which continues to be accused of human rights abuses.
"There will be no tolerance for wasting a cent of the money of our people," Mr Saied said.
He vowed to protect freedom, saying: "Those who have nostalgia to go back to the old years only pursue illusions and mirages."
Tunisia’s president controls foreign and defence policy, governing alongside a prime minister chosen by Parliament who has authority over domestic affairs.
But Mr Saied said he envisaged a bigger state role in the economy.
"We are moving from frustration to construction and work," he said.
About 90 per cent of Tunisians aged between 18 and 25 voted for Mr Saied, compared with 49.2 per cent of those older than 60, consultancy Sigma Conseil said.
Tunisia’s youths have been a common theme in Mr Saied’s campaign.
The electoral commission said voter turnout was 55 per cent.
