Voters in Tunisia are choosing their first freely elected president in a run-off election seen as a landmark in the country's move to democracy.

Beji Caid Essebsi, who won the first round with 39% of the vote, is challenging interim leader Moncef Marzouki.

Mr Essebsi represents the secular-leaning Nidaa Tounes party.

Tunisia was the first country to depose its leader in the Arab Spring and inspired other uprisings in the region.

Mr 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. 

Security has been tightened for the election

Beji Caid Essebsi, who leads a secularist party, has support in the wealthy, coastal regions

His 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.

After casting his ballot, Mr Marzouki said Tusinians "should be proud" of themselves "because the interim period has come to a peaceful end".

Voting is well underway at a school-turned-polling station in the Tunis suburb of Kram, not too far from the presidential palace in Carthage.

At least 10 members of the Tunisian security forces are deployed here, checking bags and ID before people are allowed to enter.

Many voters have brought their children, who are playing in the courtyard while the parents are queuing. Slightly more than two hours after voting started turnout here was under 20%.

The process is being scrutinised not just by international election observers, but also by thousands of Tunisian observers, who are walking around in blue vests and filling in forms.

"I am not just proud, I am very, very proud," said a 65-year-old man who has just arrived. "I never voted under dictatorship, this is the first time for me." 

Presidential powers

Mr Marzouki is likely to attract support from the moderate Islamist Ennahda party, which has played 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.

An interior ministry spokesman dismissed the video, saying the group "mean nothing to us".

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.