At least 12 people have been killed in an explosion targeting a bus carrying presidential guards in the Tunisian capital, the interior ministry said, calling the incident a "terror act".

The bus was hit on the busy Mohamed 5 Boulevard in central Tunis on Tuesday evening.

President Beji Caid Essebsi declared a state of emergency after the attack and said the country is "at a state of war". A curfew will be in place in the Tunis region until 5am on Wednesday morning.The President also declared one month of mourning.

Tunisia has become the target of terrorists. In June, the Islamic fighters attacked coastline crowded with tourists, and opened fire. 39 people were killed in a terrorist attack.