A suicide bomber has killed 12 presidential guards in downtown Tunis

The government has declared a state of emergency to deal with this third serious terrorist attack this past year.

Tunisian President Beji Caid Essebsi calls the bombing a painful event and as great tragedy, vowing his government will win its war against terrorism.

The explosion took place on a tree-line avenue in the heart of the Tunisian capital.

The bomber detonated his belt aboard a bus carrying presidential guards to the palace on the outskirts of town.

Seventeen people were injured.

Attacks outside a downtown museum in March and at the beach resort Souse last June have killed 60 people and brought Tunisia's vital tourism industry to its knees.