With 27% of votes counted, the independent election commission said conservative law professor Kais Saied and detained media magnate Nabil Karoui were leading, with a moderate Islamist candidate slightly behind.
If their lead holds, it would represent an earthquake in Tunisian politics and a strong rejection of successive governments that have struggled to improve living standards or end corruption.
Karoui has for years used his Nessma television station and the charity he founded after his son died to present himself as a champion of the poor and a scourge of government, while his critics describe him as an ambitious, unscrupulous, populist.
He denies all claims of wrongdoing against him, including old tax evasion and money laundering charges for which he was detained late last month and spent election day in jail, calling them an undemocratic plot.
His wealth and massive electoral organisation stand in sharp contrast to Saied, who spent so little on his campaign that Tunisians joke it cost no more than a coffee and packet of cigarettes.
Saied, who always speaks as if in a faculty meeting in didactic, ultra-correct formal Arabic, drives a shabby old car and wants to remain in his humble house if elected rather than move into the luxurious presidential palace at Carthage.
A social conservative who backs restoring the death penalty and rejects equal inheritance for men and women, Saied's main focus is decentralisation in a country where politicians in the capital have traditionally dominated.
At 9.30am, Saied was on 19%, Karoui was in second place with 15% and the moderate Islamist Ennahda party candidate Abdelfattah Mourou was on 13%, the official figures showed.
ISLAMISTS, POPULISTS, ESTABLISHMENT
Saied on Sunday described his lead as "like a new revolution" in a radio interview, a reference to Tunisia's 2011 uprising that brought in democracy and set off the Arab Spring revolts elsewhere.
Tunisia's prime minister, two former prime ministers, the defence minister and a former president were among the political heavyweights competing, along with Ennahda's Mourou.
"We received the message sent by the Tunisian people," Prime Minister Youssef Chahed said late on Sunday, conceding defeat.
Banned before the revolution and once seen as the main anti-establishment force in Tunisia, Ennahda has been a big player in successive coalition governments caught between the public desire for more spending and a need to reduce debt.
Elections for parliament - which has more power than the president - are coming on Oct. 6 and an Ennahda official said the party was now focused on this, and was still committed to sharing power with other major parties.
With a low turnout of 45% - down from 63% in 2014 - the result underscored widespread frustration over the sluggish economy, high unemployment, poor public services and persistent corruption.
Karoui, in a message read by his wife after the exit polls were published, said it was a message to a political elite that he accuses of using the judicial process to try to silence him.
If early results hold, and he now advances to a run-off, it could cause a constitutional headache for Tunisia's governing institutions.
The electoral commission has said he can stay in the race so long as he has not been convicted, though no final verdict in his case seems imminent.
Meanwhile, successive courts have ruled that he must stay in custody while facing the charges, despite complaints by election monitors that this prejudices his chances. His opponents say his use of his unlicensed television station should itself disqualify him.
It is also unclear if he would legally become president if unable to attend a swearing in ceremony while stuck in jail, or whether presidential immunity from prosecution would apply in an existing case.
A constitutional court that would normally address such issues has still not been set up.
"You punished those who tried to steal the votes by putting me in prison without trial, and who prevented me from speaking to people in the campaign," Karoui's Sunday night message said.