Seven million Tunisians are expected to head to the ballot box in the September 15 poll, which was brought forward from November following Essebsi's death in July.

Tunisia's first democratically elected president, Essebsi took office in the wake of the 2011 uprising that overthrew former dictator Zine El Abidine Ben Ali and sparked revolts across the Arab world.

The North African country has been praised as a rare case of democratic transition after the Arab Spring.

But it has faced economic woes and repeated jihadist attacks -- including a gun battle in the northwest on Monday that killed a national guard officer and three suspected jihadists.

Prime Minister Youssef Chahed, among those vying for the presidency, condemned the attack and vowed to raise the security budget by 15 percent.

The vote comes with Tunisia's political class divided by internal conflicts, including a struggle between Chahed and Essebsi's son, leading the premier to quit the ruling Nidaa Tounes party and form Tahia Tounes.

The latter has since become the second largest party in parliament, behind the Islamist-inspired Ennahdha party.

Aged 43, Chahed is Tunisia's youngest prime minister, but since he took office his popularity has been hit by instability, inflation and unemployment.

Other candidates include interim parliamentary speaker Abdelfattah Mourou, of Ennahdha, and former defence minister Abdelkrim Zbidi.

Another powerful opponent could be media magnate Nabil Karoui, who despite his arrest on August 23 for alleged money laundering is still legally allowed to run.

That makes for a vote markedly different from the 2014 election which had just two frontrunners -- Essebsi and former president Moncef Marzouki.

"It is the first time that Tunisians have no idea who will become their president," said the Tunis-based Joussour think-tank.

"Now everything is possible."

- Karoui controversy-

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Chahed launched his overseas campaign Saturday in the French city of Lyon, casting himself as a human rights defender. Some 1.2 million Tunisians live overseas, mainly in Europe.

He told AFP he had a "vision for a stronger Tunisia, a more modern and developed Tunisia, and a Tunisia that truly believes in the universal values of human rights," he told AFP ahead of a campaign meeting.

On Monday he called for a more market-based economy in a country where several major state firms are facing financial and management woes.

"Schools and health services are part of the state's role but there are many areas in which the state will no longer intervene," he told supporters in Tunis.

Chahed has also denied accusations from Karoui backers that he had masterminded the arrest of the controversial media magnate.

Karoui has said he was targeted by "attempts to undermine his growing popularity".

In recent years, Karoui used his popular Nessma TV, which is now banned from covering the election, to launch high-profile charity campaigns.

Chahed's government tried to eliminate Karoui from the race, passing an amended electoral code through parliament -- but Essebsi died without ratifying it.

Tunisia's electoral commission, which on Saturday approved the final list of 26 hopefuls, has banned the publication of opinion polls, making it difficult for pundits to evaluate each candidate's strength.

- 'Power cannot be shared' -

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Zbidi told AFP that if he were elected he was determined to "restart the social ladder" and make public services accessible for all Tunisians.

The former defence minister called for constitutional changes to give the head of state more powers.

"Power cannot be shared," he said.

But he said a referendum should be held to choose between a "parliamentary regime or a presidential one".

Ennahdha's candidate Mourou is a founding member of the Islamist party and the first presidential candidate it has ever fielded.

Political analyst Hamza Meddeb said he believed Mourou would make it to the second round of voting expected to be held in the first week of November.

"The big unknown is who will be his rival," Karoui, Chahed or Zbidi, he said.

The final list of candidates includes only two women -- former tourism minister Salma Elloumi and Abir Moussi, who heads a group formed from the remnants of Ben Ali's ruling party.

Campaigning ends on September 13, two days ahead of voting.