Saied, 66, is expected to win by a landslide, routing his challengers -- imprisoned rival Ayachi Zammel, who was set to collect 6.9 percent of the vote, and Zouhair Maghzaoui, with 3.9 percent, said independent polling group Sigma Conseil.
Three years after Saied staged a sweeping power grab, rights groups fear re-election will only further entrench his rule in the country, the only democracy to emerge from the Arab Spring uprisings.
With the ouster of longtime dictator Zine El Abidine Ben Ali in 2011, Tunisia prided itself on being the birthplace of those regional revolts against authoritarianism.
But the north African country's path changed dramatically soon after Saied's election in 2019.
The Tunisian electoral board, ISIE, has said about 9.7 million people were eligible to vote, in a country whose population is around 12 million.
Only 27.7 percent of voters turned out to cast their ballots, it said. Over 58 percent were men, and 65 percent aged between 36 and 60.
"The vote's legitimacy is undoubtedly tainted with candidates who could have overshadowed (Saied) being systematically sidelined," said Hatem Nafti, a political commentator and author of a forthcoming book on the current president's authoritarian rule.
ISIE had barred 14 candidates from joining the race, citing insufficient endorsements, among other technicalities.
Speaking at his campaign's office in the capital, Saied warned of "foreign interference" and pledged to "build our country and we will rid it of the corrupt and conspirators".
"The results announced by the exit polls are very close to reality," he told national television. "We will wait for the official results."
The board is set to announce the preliminary election results on Monday.
'Weak legitimacy'
This year's turnout figure compared to 45 percent in 2019 and is the lowest the country has recorded in a presidential vote since its 2011 revolution.
The vote's "democratic legitimacy is indeed weak, but there is no minimum threshold," said north Africa analyst Pierre Vermeren. "The majority of Tunisians let it happen."
Hosni Abidi, 40, said he feared fraud after the electoral board also barred two local independent watchdogs from monitoring the vote.
"I don't want people to choose for me," he said. "I want to check the box for my candidate myself."
Saied cast his vote alongside his wife in the affluent Ennasr neighbourhood, north of Tunis, in the morning.
Shortly after the exit polls were announced, hundreds of supporters took to the street celebrating his expected win.
"I am convinced by his ideas and his politics," said Oumayma Dhouib, 25.
Saied's 2021 power grab saw him rewrite the constitution and crackdown on dissent, sparking criticism at home and abroad.
New York-based Human Rights Watch has said more than "170 people are detained in Tunisia on political grounds or for exercising their fundamental rights".
His top challenger, Zammel, currently faces more than 14 years in prison, accused of having forged endorsement signatures to enable him to stand in the election.
Other jailed figures include Rached Ghannouchi, head of the Islamist-inspired opposition party Ennahdha, which dominated political life after the revolution.
Also detained is Abir Moussi, head of the Free Destourian Party, which critics accuse of wanting to bring back the regime that was ousted in 2011.
'Pharaoh manipulating the law'
In a speech on Thursday, Saied called for a "massive turnout" to usher in what he called an era of "reconstruction".
He cited "a long war against conspiratorial forces linked to foreign circles", accusing them of "infiltrating many public services and disrupting hundreds of projects" under his tenure.
"Many fear that a new mandate for Saied will only deepen the country's socioeconomic woes, as well as hasten the regime's authoritarian drift," the think tank said.
Wajd Harrar, a 22-year-old student, said that in 2019, while she was too young to vote, "people had chosen a bad president".
This time, she said, "I have the right to vote and I will give my vote to the least bad candidate."
On Friday, hundreds of people protested in the capital, some holding signs denouncing Saied as a "Pharaoh manipulating the law".
