Behind the landslide victory of conservative political outsider Kais Saied in a run-off presidential vote were millions of young voters, who describe him as a leader worthy of their trust as Tunisia's democracy takes root.

According to a poll by the Sigma polling institute, around 90 per cent of voters aged between 18 and 25 voted for political newcomer Mr Saied, massively shunning his rival, business tycoon Nabil Karoui.

In contrast, the institute said 49.2 per cent of voters aged more than 60 voted for the winner.

Under Tunisian law, the results can be appealed within two weeks before the new president is sworn in at the end of the month.

Mr Saied, a 61-year-old constitutional law expert, campaigned on a platform of opposition to Westernised and corrupt elites and in favour of radical decentralisation.

Tunisia's second free presidential election since the Arab Spring in 2011 followed President Beji Caid Essebsi's death in July.

However, he had said he would not run for a second term. In his first reaction as exit polls showed he was the clear victor, Mr Saied thanked the country's young people "for turning a new page" and vowed to try to build "a new Tunisia".

"In the first round, I didn't think he would win. But now that he has won I can truly believe in the transparency of our elections," 20-year-old law student Mayssa Jlassi said.

"We had to do everything possible to mobilise all young people like me to vote en masse" for Mr Saied, she added.

Mr Jlassi joined a volunteer group that went door-to-door, with very modest means, to canvass for votes for her candidate.

Overall, Mr Saied, a 61-year-old retired law professor, scooped 72.71 per cent of votes, official results showed.

"The main reason for his victory was the extraordinary mobilisation of young people aged between 18 and 25," said Olfa Lamloum, director of the Tunis branch of the NGO International Alert, which works closely with young people.

According to Sigma, 37 percent of young voters cast their ballot for Mr Saied in the September 15 first round — twice the national average — while the disparity was magnified further in Sunday's runoff.

Despite having spearheaded the Arab Spring revolt that ousted dictator Zine El Abidine Ben Ali in 2011, young people had largely shunned the ballot box in previous elections in Tunisia's new democracy.

This time was different

Mr Saied "managed to win the trust of young people...Not with promises but by offering answers to the failures of representative democracy," Mr Lamloum said.

The anti-establishment Saied is seen as uptight and unwavering, and beneath his austere style is a commitment to socially conservative views that many young people reject.

But he has said he will respect the social freedoms enshrined in law in recent years that civil society groups have hailed as freedoms.

And what his young supporters see in him above all is an honest leader who is offering them the keys to building Tunisia's future.

Mr Saied has promised "to challenge the top-down nature of power and to change the rules of the game of politics, which are the root cause for the exclusion of young people," Mr Lamloum said.