Retired academic Kais Saied is set to become president of Tunisia, exit polls suggest.
The poll said the law professor, 61, had secured 76% of votes. No official results have yet been published.
He was up against media mogul Nabil Karoui, 56, who campaigned from prison after being arrested on charges of money laundering and tax fraud.
He denies the charges and officials have warned he may appeal, arguing he was not allowed a fair campaign.
After the exit polls were released, Mr Saied appeared in front of jubilant supporters in Tunis and thanked “young people for turning a new page” in Tunisia’s history.
“We will try to build a new Tunisia. Young people led this campaign, and I am responsible for them,” he said, flanked by his family.
Mr Karoui, who was freed by a court order just four days ago, came second in the first round of voting, with 15.6% of the vote to Mr Saied’s 18.4%.
Turnout exceeded 50%.
With Kais Saied apparently leading by a large majority, his supporters took to the streets in celebration. Official results will be revealed on Monday.
Kais Saeid and his rival Nabil Karoui held a debate on Friday night that was watched by more than six million Tunisians.
Mr Saied provided little policy detail, merely speaking of empowering the people and being a man of the law, leaving his critics and undecided voters with the impression that he has no plan.
Mr Karoui, meanwhile, said he would alleviate poverty and bring in high-tech jobs from Google, Netflix and Amazon – a promise that was the subject of much satire on social media.
The debate, however, did appear to inject more energy into the second round of voting.
