According to the Bawaba news website, 15 people were killed and several others injured in the attack.
It is said to have been the work of a suicide bomber.
Ambulances rushed wounded from the scene and security forces closed off streets around Mohamed V Avenue, one of the main streets in Tunis, before the president declared a curfew in the city and imposed a state of emergency nationwide.
The bombing happened an hour before a planned speech President Beji Caid Essebsi.
“They want to make us live with horror but we are going to bring that horror to the terrorist camps,” President Beji Caid Essebsi said in a televised speech. “We are at war and we are going to win.”
It was the third major attack in Tunisia this year, after a terrorist killed 38 foreigners at a beach hotel in the resort of Sousse in June, and gunmen killed 21 tourists at the Bardo Museum in Tunis in March. ISIL claimed both those attacks, too.