Essebsi, a former Ben Ali official, beat rival Moncef Marzouki with 55.68 percent of the vote against 44.32 percent in Sunday's run-off ballot between the two men, according to results released on Monday by electoral authorities.

Both candidates called for calm after rioting briefly broke out in several southern towns in protest over the return of an old guard figure, and witnesses said an office of Essebsi's Nidaa Tounes party was set alight in one town.

A former parliament speaker under Ben Ali, Essebsi recast himself as an experienced technocrat. His secular party Nidaa Tounes - Call for Tunisia - profited from a backlash against the post-revolt Islamist government, which many voters blamed for turmoil after 2011.

"I will be president for all Tunisians," Essebsi said in a brief speech on state television.

Outside the Nidaa Tounes headquarters in the capital Tunis, several hundred jubilant supporters took to the streets to celebrate with flares and music, waving Tunisia's red and white national flag and honking car horns.

"He's the right man for the right time," said government finance specialist Sana Ben Said at the Essebsi rally.

In a short television address Marzouki accepted his defeat despite what he said were suspected vote irregularities, which he would not challenge.

Just after results were announced, protesters set fire to a Nidaa Tounes party office in Tataouine in southern Tunisia, witnesses said. Residents said rioting broke out in several southern towns, including Hamma, where police firing teargas to disperse hundreds of youths who burned tyres to demonstrate.

Critics of Essebsi, an 88-year-old who has spent five decades in Tunisian politics, see his return as a setback for the 2011 uprising that ousted Ben Ali and put the North African country on the road to full democracy, with a new constitution and free parliamentary and presidential elections.

The Tunisian uprising also inspired the Arab Spring revolts across North Africa and the Middle East.

U.S. President Barack Obama and U.S. Secretary of State John Kerry congratulated Essebsi on his victory and Tunisia on its conclusion of the first presidential election under the new constitution, according to U.S. statements.

"Tunisia has provided a shining example to the region and the world of what can be achieved through dedication to democracy, consensus, and an inclusive political process," Kerry said.

OLD GUARD RETURN?

Compromise has been important in Tunisian politics. Essebsi's Nidaa Tounes reached a deal with the Islamist Ennahda party to overcome a political crisis triggered by the murder of two secular leaders last year.

As frontrunner, Essebsi dismissed critics who said victory for him would mark a return of the old guard. He distanced himself from the corruption and abuses of Ben Ali's era, and argued that he was the statesman Tunisia needed following three messy years of an Islamist-led coalition government.

Marzouki, who had sought refuge in France during the Ben Ali era, painted a potential Essebsi presidency as a reverse for the "Jasmine Revolution" that forced the former autocrat to flee into exile. But many tied Marzouki to the Ennahda government.

Accepting former regime officials - known as the "Remnants" by their critics - back into politics had been one of the compromises that helped restore calm and keep Tunisia's often unsteady transition to democracy on track.

Victory for Essebsi consolidates his secular Nidaa Tounes party's position after it won the most seats in October's parliamentary election, beating out Islamist party Ennahda.

Essebsi's party will now have to decide on a prime minister and form a new government in coalition with smaller parties in the parliament before tackling Tunisia's low-level Islamist militancy and politically sensitive economic reforms.

Ennahda holds the second largest number of seats in congress and it and leftist Popular Front movement are still strong movements that will prove tough opponents in negotiations over policy in parliament.