Saied's supporters exploded with joy, celebrating on the main boulevard of Tunis even though official results from Sunday's runoff vote aren't expected until Tuesday. His opponent, Nabil Karoui who was in jail for most of the campaign, was forecast to win between 23 and 27 percent.
Karoui, the media mogul, was only released from detention on Wednesday after spending most of the election campaign behind bars awaiting a verdict in his corruption trial.
They trounced the old guard in a September 15 first round, highlighting voter anger over a stagnant economy, joblessness and poor public services in the cradle of the Arab Spring.
Last week, Tunisia held its legislative elections, in which Karoui's Heart of Tunisia party came second with 38 seats, while the Ennahda movement has emerged as the strongest force with 52 seats in the 217-member parliament.
The victor will inherit a country struggling to revive tourism, create jobs and stem Islamic extremism.
The publication of the exit polls triggered celebrations at Saied's election campaign offices in central Tunis, as fireworks were set off outside and supporters honked auto horns.
Opposite the polling station, Othman Dabbousi, 63, sat next to his fruit stand and said he would not vote, preferring a non-democratic system of rule.
"This rivalry for Carthage palace (the presidency) is more exciting than a local derby (football match)", said a young man taking part in lively debate on Bourguiba Avenue in central Tunis ahead of the showdown.
His arrest in the run-up to the election cemented his status as an outsider - despite being a longtime key supporter of Essebsi, whose death on July 25 brought forward the polls.
He sits poker-straight, his blank visage hiding any visible sign of emotion, and speaks in a staccato style - and in literary Arabic, a tongue inaccessible to many in Tunisia's rural interior.
About 7.2 million people were registered to vote, with more than 4,500 polling stations across the country.
During an unprecedented TV debate, Karoui promised to combat extremist violence by "attacking at its roots" and raising economic prospects in struggling provinces that are fertile recruiting grounds for the Islamic State group and other extremists. In addition to economic and security troubles, Tunisia is both a source of migrants trying to reach Europe and a transit country for migrant trafficking from elsewhere in Africa.
Tunisians started voting on Sunday for a new president in a second and final round of the country's presidential election.
However, several political and social parties have expressed concerns about low turnout just like what happened in the first round of the presidential race and in the parliamentary elections.