"Unfortunately the indictment chamber... refused the release request," lawyer Kamel Ben Messoud told AFP, after what was the fourth such appeal turned down by the judiciary in the North African country.

A leader in Karoui's Qalb Tounes party, Oussama Khlifi, told local media that his continued detention "threatens the democratic process".

The 56-year-old, who has been under investigation since 2017 for money laundering and tax evasion, was placed in pre-trial detention on August 23 and subsequent applications to release him have been denied.

The timing of his arrest, 10 days before the start of campaigning, raised questions about the politicisation of the judicial process.

In July, an investigating judge froze the assets of Karoui and his brother Ghazi, and imposed a travel ban.

When Karoui appealed against those decisions, the indictment chamber issued an arrest warrant, and he was detained at a motorway tollbooth after an election visit.

Despite the legal proceedings, Karoui's candidacy was approved by the elections commission ISIE and he campaigned via the Nessma television channel he founded and through his wife Salwa Smaoui.

After the first round of voting on September 15, independent law professor Kais Saied led with 18.4 percent of votes, according to ISIE, with Karoui in second with 15.6 percent.

Karoui's party, alongside ISIE, international observers and various politicians have called for the jailed candidate to be allowed to campaign fairly ahead of the second round, expected on October 13.

The International Crisis Group has warned that the continued detention of Karoui "jeopardizes the entire electoral process" and that "the courts could cancel the entire vote".

The candidate's spokesman called for the vote to be suspended while the candidate is in prison.

"The second round will take place when Nabil is free," Hatem Mliki told AFP. "Our political opponents are trying to use every means... to prevent a peaceful transfer of power."

Karoui's election rival, Saied, has also called for him to be freed to contest the polls.

Rival 'morally uncomfortable'

"The situation leaves me morally uncomfortable... I would sincerely like to see him freed," Saied told national television last week.

Tunisian television is due to broadcast a debate between Karoui and Saied as part of the election campaign.

Both ISIE and broadcast regulator HAICA have requested that the judiciary authorise Karoui to take part in that debate.

State television has said it is prepared to organise the debate within prison walls if necessary.

Karoui has remained eligible to run despite his imprisonment, as long as any conviction does not also specifically deprive him of his civil rights, according to ISIE.

Tunisia's presidential vote was brought forward after the death of the incumbent, Beji Caid Essebsi, on July 25. Parliament speaker Mohamed Ennaceur took over on an interim basis for a 90-day period.

Under the constitution, Ennaceur has to make way for an elected president by October 23.

Millions of Tunisians are also to head to ballot boxes on October 6 to elect parliamentary representatives in a key vote which is however being overshadowed by the presidential polls.