TUNIS - Tunisians go to the polls Sunday 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 pits 88-year-old favourite Beji Caid Essebsi, leader of the anti-Islamist Nidaa Tounes party, against incumbent Moncef Marzouki, who held the post through an alliance with the moderate Islamist movement Ennahda.
Ahead of the landmark second-round vote, which sets Tunisia apart from the turmoil of other Arab Spring countries, jihadists issued a videotaped threat against the North African state's political establishment.
The vote is the first free presidential election since independence from France in 1956.
Amid tight security and the closure of main border posts with strife-torn neighbour Libya, almost 5.3 million Tunisians are eligible to vote.
Polling stations are open from 8:00 am (0700 GMT) until 6:00 pm (1700 GMT).
Citizens living abroad began voting on Friday.
A first round held on November 23 saw Essebsi win 39 per cent of the vote, six per cent points ahead of Marzouki, a 69-year-old former rights activist installed by parliament two months after December 2011 polls.
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 Spring.
Throughout the campaign, Marzouki has accused Essebsi, who served as a senior official in previous Tunisian regimes, 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.
The final result is expected to be announced between December 22 and 24.
In an Internet video posted on Wednesday night, jihadists claimed the 2013 murder of two secular politicians that plunged Tunisia into crisis, warning of more killings of politicians and security forces.
Last year's murders had threatened to derail Tunisia's post-Arab Spring 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 jihadist threat, Tunisia faces 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 Arab Spring countries 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."