Two political outsiders are set to go head-to-head in the race to become Tunisia's next president, according to the latest count after a first round of voting on Sunday.

If confirmed, the outcome would be a blow to the political class that has ruled since 2011 in the birthplace of the Arab Spring revolts.

Why were established candidates sidelined?

Tumbling turnout, down from 64% in Tunisia's first democratic polls in 2014 to just 45% on Sunday, was a reflection of voters' fatigue with political infighting, analysts said.

"The abstention was a sign of a rejection of the system rather than disinterest," said political scientist Hamza Meddeb.

"People are fed up with a political class which failed to respond to their economic and social expectations," he said, criticising candidates for their "lack of thought and vision".

Sophie Bessis, a historian, said senior politicians had failed to convince voters that they would "resolve their basic problems".

The two frontrunners - law professor and independent Kais Saied and jailed media magnate Nabil Karoui - cornered the "anti-system vote", despite the latter being seen as part of Tunisia's "elite", Meddeb said.

What happens if Karoui contests runoff?

The media magnate is under provisional detention on money laundering charges, increasing the pressure on the judiciary to release him if he goes through to a second round.

That would place the country "in a judicial impasse", said the deputy prosecutor general of the Tunis appeals court, Ibrahim Bouslah.

But "whether he remains in prison or is released, problems remain: he has not been tried and the case is not closed," he added.

"If he reaches the presidency, he would not have presidential immunity because that is not retroactive."

Medleb said the judiciary could even disqualify Karoui if it opted to broaden its investigation and examine possible infractions of party financing rules.

However, according to electoral commission member Mohamed Tlili Mansri, a jail term would not prevent Karoui from running or even serving as president, unless his sentence specifically strips him of civic and political rights.

What are consequences for Tunisian politics?

Sunday's vote came just three weeks before the country's October 6 legislative election.

If Karoui does go forward, it could benefit his Qalb Tounes party, leaving open the question of "where the pro-Kais Saied vote goes," said Meddeb.

Another vote in protest at established parties could however benefit independent lists.