Tunisians are choosing between a retired law professor and a media mogul in the final vote of a presidential election on Sunday, eight years after a revolution that forged a new democracy and inspired the "Arab spring".
The election was held early following the July death in office of President Beji Caid Essebsi.
A Tunisian presidential election exit poll conducted by Sigma Consulting projected a landslide win for independent candidate Kais Saied on Sunday, saying he had taken 76.9% of the votes.
The two candidates offered starkly different options for president: Saied has conservative social views, and wants Tunisia to adopt an experimental form of direct democracy.
Enigmatic former constitutional law professor Kais Saied spent nearly no money on his campaign and is regarded by his supporters as a humble man of principle, while his critics have attacked his conservative social views and backing by the moderate Islamist party Ennahda.
On Friday night, Karoui and law professor Kais Saied went head-to-head in a rare television debate, a last bid to woo voters.
Polling stations open at 8:00 am (0700 GMT) for the runoff finale, with 12 hours of voting to decide the North African country's next leader.
Saied topped the first round in the presidential election, held on September 15, with 18.4 percent of votes while Karoui followed with 15.6 percent.
The 56-year-old business tycoon Karoui portrayed himself as a bulwark against political Islam, which he accused his rival of supporting.
For what was the third election in four weeks, some Tunisians organised car-sharing and free transport for students who had to travel far to their hometowns to cast their ballots.
While the candidates are both seen as anti-establishment figures, the contrast between them is sharp, with Saied nicknamed "Robocop" for his rigid and austere manner. He has called for the criminalization of homosexuality and a sexual assault law that reprimands unmarried couples who publicly display affection.
"We need to renew confidence between the people and the rulers", Saied said in televised comments after two separate exit polls gave him more than 70% of the vote.
News of the victory triggered celebrations at the retired law professor's election campaign offices in central Tunis, as fireworks were set off outside and supporters honked vehicle horns. He has continued to deny the allegations against him and says they are politically motivated.
In a contest which reflected Tunisia's shifting post-revolution political landscape, Saied, an independent, scooped nearly 77 percent of the vote, compared to 23 percent for Karoui, Wataniya television said.
Although now a free man, Karoui is still under investigation for fraud and banned from travelling overseas.
A former executive for Colgate-Palmolive, in 2002 Karoui launched a media agency with his brother.