Secular-leaning Nidaa Tounes party’s candidate Beji Caid Essebsi, who won the first round with 39% of the vote, is challenging interim leader Moncef Marzouki.
Tunisia was the first country to depose its leader in the Arab Spring and inspired other uprisings in the region.
Beji Caid Essebsi, who turned 88 this week, held office under both deposed President Zine el-Abedine Ben Ali and Tunisia’s first post-independence leader, Habib Bourguiba.
He is popular in the wealthy, coastal regions, and has based his appeal to voters on stability and experience.
Beji Caid Essebsi’s opponent, Moncef Marzouki, is a 67-year-old human rights activists forced into exile by the Ben Ali government.
He has been interim president since 2011 and is more popular in the conservative, poorer south.
Moncef Marzouki is likely to attract support from the Islamist Ennahda party, which has paid a key role in Tunisian politics since the Arab Spring but has not fielded a candidate.
Whoever wins faces restricted powers under a constitution passed earlier this year.
The president will be commander-in-chief of the armed forces but can appoint or sack senior officers only in consultation with the prime minister.
The president will also set foreign policy in consultation with the prime minister, represent the state and ratify treaties.
Tunisia has boosted security for the elections and closed border posts with Libya, which has been plagued by unrest.
In the build-up to the vote, a video emerged of Islamic State militants claiming responsibility for the 2013 killings of two Tunisian politicians.
The men in the video also condemned the election and threatened more killings.
About 5.2 million Tunisians are eligible to vote in the run-off poll. At least 88,000 observers are overseeing the election, according to Tunisian state media.
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