Nearly 30 candidates are running, but a race is emerging between a veteran ex-Ben Ali official and a rights activist who says the election is a chance to stop the return of old-regime stalwarts.

More than three years after the end of Ben Ali's one-party rule, Tunisia has become a model of transition for the region by adopting a new constitution and avoiding the turmoil facing its neighbors.

Sunday's vote follows an October general election when the main secular Nidaa Tounes party won the most seats in the parliament, besting the Islamist party Ennahda that won the first free poll in 2011, Reuters reported.

Compromise between secular and Islamist rivals has become the byword for Tunisia's political success, but the ascent of former regime officials is worrying critics who say they fear their return will be a setback for the 2011 revolution.

Nidaa Tounes leader, Beji Caid Essebsi, an 87-year-old former Ben Ali official, is emerging as a frontrunner along with main rival, current President Moncef Marzouki, who is warning against the rise of one-party era figures like Essebsi.

"The old regime wants to impose itself on these elections especially after they won the most seats in the parliament," said bank employee Mohammed Souilmi.

"I will be voting to put a stop to that."

Essebsi and other former Ben Ali era officials say they are not tainted by the corruption and abuse of the former regime. They present themselves as technocrats with the skills to help Tunisia.