Tunisia's Islamist party is denouncing a deadly attack on the country's presidential guard and urging 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.

Tunisia's president has declared a state of emergency throughout the country and a curfew in the capital after an attack on a bus carrying his presidential guard that killed 12 people.

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." He wasn't in the bus when it was attacked Tuesday in the center of the capital.

His office says he is cancelling a trip to Switzerland that had been scheduled for Wednesday.