Tunisians are going to the polls to freely elect their president for the first time in the country's history, rounding off an at times bumpy four-year transition from dictatorship.

The runoff on Sunday pits 88-year-old Beji Caid Essebsi, leader of the Nidaa Tounes party, against incumbent Moncef Marzouki, who held the post through an alliance with the moderate Islamist movement Ennahda.

Al Jazeera's Jamal ElShayyal reporting from Tunis:
The turnout so far is still below 20 percent from what we are hearing from the election commission. However, it is expected to pick up as the day goes. It is important to know that there are a number of factors that could result in a lower-than-expected turnout.

There is the fact that there is a bit of election fatigue, so to speak, as this is the third time that Tunisians are expected to go to the polls in less than two months.

There’s also maybe some sense of fear in terms of the security situation after [an] ISIL video that was posted earlier that was threatening people not to take part in the election and threatening to attack institutions, although the government has tried their best to secure the polling stations and dispatch thousands of troops across [the country].

Nonetheless, it is a very tightly contested race… The age factor has been the centre of this issue. One of the candidates, Beji Caid Essebsi, is 88 years old and considering that a huge part of the Tunisian population is under the age of 35 years old and considering that it was the youth who are the forefront of change here, this is going to be a significant factor in making people’s minds up.

The landmark second-round vote sets Tunisia apart from the turmoil of other Arab countries that went through the uprisings that started in 2011.

The vote, which is taking place amid tight security and the closure of main border posts with strife-torn neighbour Libya is the first free presidential election since independence from France in 1956.

Almost 5.3 million Tunisians are eligible to vote, starting from at 8am (0700 GMT) until 6pm (1700 GMT).

Concerns about low voter turnout persisted, however, after the first round which had a low number of young voters.

This was widely blamed on the political parties, which remain unable to engage the youth and address their issues.

A first round, held on November 23, saw Essebsi win 39 percent of the vote, six percent points ahead of Marzouki, a 69-year-old former rights activist installed by parliament two months after December 2011 polls.

Election day was marred by violence as Tunisian troops killed a gunman and captured three others after they attacked soldiers guarding ballot papers, the defence ministry said.

The pre-dawn attack targeted a school in the central region of Kairouan where the ballot papers had been stored under army guard.

"The vigilance of the soldiers and the swiftness of their response thwarted this operation and led to the death of a man armed with a hunting rifle and the arrest of three suspects," ministry spokesman Belhassan Oueslati told AFP news agency.

The vote is the country's third in as many months, after Nidaa Tounes won an October parliamentary election, making Essebsi favourite to be the next president, but with powers curbed under constitutional amendments to guard against a return to dictatorship.

The campaign has been marked by mudslinging, with Essebsi refusing to take part in a debate with Marzouki, claiming his opponent is an "extremist".

Essebsi insists that Marzouki represents the Islamists, charging that they had "ruined" the country since the 2011 revolution which toppled veteran ruler Zine El Abidine Ben Ali and gave birth to the Arab uprisings.

Throughout the campaign, Marzouki has accused Essebsi, who served as a senior official in previous Tunisian governments, of wanting to restore the old guard deposed in the revolution.

He has even suggested that Essebsi's camp was preparing to "win through fraud", drawing a sharp rebuke from the electoral commission.

Video threat

The final result is expected to be announced between December 22 and 24.

In an internet video posted on Wednesday night, armed men claimed the 2013 murder of two secular politicians that plunged Tunisia into crisis, warning of more killings of politicians and security forces.

Al Jazeera's Tunisia coverage:

Last year's murders had threatened to derail Tunisia's post-Arab uprisings transition until a compromise government was formed in January this year.

The authorities have deployed tens of thousands of troops and police to guarantee security on Sunday.

In addition to the threat, Tunisia faces other major challenges.

Its economy is struggling to recover from the upheaval of the revolution, and there are also fears of widespread joblessness causing social unrest.

The International Crisis Group think-tank has said Tunisia was the "last hope" for a peaceful transition to democracy, setting it apart from other Arab countries that went through uprisings, such as Libya and Egypt.

"In the context of the meagre harvest of the Arab Spring, Tunisia remains the last hope for a successful democratic transition," it said.

"The country and its allies have every reason to ensure that Tunisia continues on its exceptional course."

Source: Al Jazeera and agencies