TUNIS: An explosion struck a bus carrying members of Tunisia's presidential guard in the country's capital Tuesday, killing at least 12 people and wounding more than a dozen others in what the Interior Ministry called a “terrorist act.”

The blast on a tree-lined avenue in the heart of Tunis is a new blow to a country that has struggled against Islamic extremist violence. Radical gunmen staged two attacks earlier this year that killed 60 people, devastated the tourism industry and rattled this young democracy.

Tunisia's president has declared a state of emergency throughout the country and a curfew in the capital after the attack.

Beji Caid Essebsi said in a televised address that the country is at “war against terrorism” and called for international cooperation against extremists who have staged several attacks around the world in recent weeks.

Essebsi said “I want to reassure the Tunisian people that we will vanquish terrorism.”

Police fanned out throughout central Tunis after Tuesday's explosion, and ambulances rushed to the scene, evacuating wounded and dead. Prime Minister Habib Essid and Interior Minister Najem Gharsalli visited the scene of the attack after it was cordoned off by police.

Interior Ministry spokesman Walid Louguini told The Associated Press that at least 12 were killed and 16 wounded in what the government considers a “terrorist act”.

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President Beji Caid Essebsi wasn't in the bus at the time. His presidential guard is an elite security force that protects only the president.

Witness Bassem Trifi, a human rights lawyer, said the explosion hit the driver's side of the bus, describing a “catastrophic spectacle.”

“I saw at least five corpses on the ground,” he told the AP. “This was not an ordinary explosion.”

Meanwhile, Tunisia's Islamist party has denounced a deadly attack on the country's presidential guard and urged Tunisians to unite behind the security forces as they hunt for the perpetrators.

The Ennahda party said in a statement that “Tunisia is targeted because it is a democracy and represents a model of moderate Islam.”

No one has claimed responsibility for Tuesday's attack, though Tunisia has been hit by two big attacks by Islamic extremists this year.

It called on all Tunisians to unite, saying “in these historic moments we should renew our trust in the security forces and army.”

The party won legislative elections after Tunisia's Arab Spring uprisings but struggled with violence and instability. It is no longer the governing party but remains a prominent political force.

The attack came days after authorities visibly but inexplicably increased the security level in the capital and deployed security forces in unusually high numbers. The French Embassy is this former French territory was also put under high protection in recent days, after extremist attacks in Paris killed 130.

Earlier this month, Tunisian authorities announced the dismantling of a cell that it said had planned attacks at police stations and hotels in the seaside city of Sousse, about 150 kilometres southeast of Tunis. Sousse was one of the targets of attacks earlier this year.

Tunisia's tourism industry has been hit hard this year following extremist attacks. Shootings at a luxury beach hotel in Sousse last June killed 38 people, mostly tourists, while in March, an attack by Islamist extremists at Tunisia's famed Bardo museum near the capital killed 22 people.

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US Secretary of State John Kerry, visiting Tunis earlier this month, pledged expanded economic and security support for Tunisia, whose popular uprising unleashed democracy movements across the region in 2011 known as the Arab Spring.

Kerry said the US and Tunisia would soon begin negotiations on a major loan guarantee and were discussing expanded military cooperation, including intelligence sharing and the possible use of drones to collect information about potential threats.

A US military team was expected in Tunisia around late November to begin those talks.

Tunisia is the only Arab Spring country to have solidified a new democracy, but it is facing serious economic and security challenges.