Among the 27 candidates, the hot favourite is former premier Beji Caid Essebsi, an 87-year-old veteran whose anti-Islamist Nidaa Tounes party won parliamentary elections last month.
Others vying for the presidency include outgoing President Moncef Marzouki, several ministers who served under former dictator Zine El Abidine Ben Ali, leftwinger Hamma Hammami, business magnate Slim Riahi and a lone woman, magistrate Kalthoum Kannou.
Some 5.3 million people are eligible to cast ballots, with tens of thousands of police and troops deployed to guarantee security amid fears Islamist militants might seek to disrupt voting.
A run-off vote will be held at the end of December if no one secures an absolute majority.
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Until the revolution, Tunisia knew only two presidents -- Habib Bourguiba, the "father of independence" from France in 1956, and Ben Ali, who deposed him in a 1987 coup.
To prevent another dictatorship, presidential powers have been restricted under a new constitution, with executive prerogatives transferred to a premier drawn from parliament's top party.
Supporters argue only he can stand up to the Islamists who first held power in the post-Ben Ali era, but critics charge he is out to restore the old regime, having served under both former presidents.
Marzouki has been hammering home the argument that he is the only leader capable of preserving the gains of the uprising, and has said today's vote is the "last stand" for the old guard.
Moderate Islamist party Ennahda, which came second in the legislative election, has not put up a candidate and has invited its members "to elect a president who will guarantee democracy".