The head of Tunisia's anti-Islamist Nidaa Tounes party, Beji Caid Essebsi, was on Tuesday leading the country's first presidential election since a 2011 revolution that sparked the Arab Spring uprisings.

But with no outright winner from Sunday's poll, in which Essebsi took a lead of six percentage points over incumbent Moncef Marzouki, a runoff will be held in December.

The battle for an expected runoff vote for the Tunisian presidency kicked off Monday even before first round results were in from a landmark post-Arab Spring election.

Veteran anti-Islamist politician Beji Caid Essebsi, whose party placed first in a parliamentary election last month, looked set to fall short of the 50 percent threshold required to win outright, his campaign team conceded.

Both leading candidates in Tunisia's first free presidential election since the 2011 revolution sparked the Arab Spring predicted a runoff as each claimed to be ahead after Sunday's vote.

The election is a milestone in the North African country where a popular uprising set off a chain of revolts that saw several Arab dictators toppled by citizens demanding democratic reform.

Tunisia's secular Nidaa Tounes party won 86 seats in last month's parliamentary election, knocking moderate Islamists Ennahda into second place with 69 seats, according to definitive official results published Friday.

The final Nidaa Tounes figure was up one on the 85 seats provisionally tallied following the October 26 poll.

Tunisia's presidential election is the "last stand" for the regime toppled in 2011, incumbent Moncef Marzouki said Wednesday, in a dig at his main rival and frontrunner Beji Caid Essebsi.

Essebsi, 87, leads the field ahead of Sunday's vote, after his anti-Islamist Nidaa Tounes party won a parliamentary election on October 26, beating the previously dominant moderate Islamist movement Ennahda.

An exiled son-in-law of deposed Tunisian president Zine El Abidine Ben Ali returned to the capital Tuesday to challenge a jail sentence for having a gun, his lawyer told Agence France Presse.

Businessman Slim Chiboub, who has been living in the United Arab Emirates since Ben Ali fled Tunisia in January 2011 during a popular uprising, had been sentenced in absentia to five years in jail for illegal possession of a firearm.

Tunisia holds its first multi-candidate presidential election on Sunday in the final stage of a post-revolution transition that has set it apart from the turmoil of other Arab Spring states.

Twenty-seven candidates are in the running, with former premier Beji Caid Essebsi, an 87-year-old veteran of Tunisian politics whose anti-Islamist party Nidaa Tounes won an October 26 parliamentary election, the hot favourite.

Tunisia's Islamist Ennahda party, which came second behind its secular rival in landmark parliamentary polls last month, said Saturday it was not backing any candidate in a forthcoming presidential election.

The party, which won 69 out of 217 parliamentary seats in October, is not fielding a candidate for the November 23 vote in the impoverished North African nation, the cradle of the Arab Spring uprisings.

The death toll from a gun attack by suspected militants on soldiers traveling in a bus in northwestern Tunisia rose to five on Thursday, the defense ministry said.

The fifth victim "had severe wounds and died," spokesman Rachid Bouhoula told AFP.

Four Tunisian soldiers were killed on Wednesday when suspected militants opened fire on a minibus carrying troops, the defense ministry said.

Describing the attack in northwestern Tunisia as a "terrorist operation," ministry spokesman Belhassen Oueslati said the suspects had escaped but were being hunted down.