Beji Caid Essebsi said in a televised address the country was at war against terrorism and called for international cooperation against extremists who have staged attacks across the world in recent weeks.
“I want to reassure the Tunisian people that we will vanquish terrorism,” he added.
His office says he is cancelling a trip to Switzerland that had been scheduled for Wednesday.
No group immediately claimed responsibility for the bombing, which a ministry official said also wounded 20 people when it went off on Mohamed V Avenue. But Tunisia has increasingly become a target for Jihadists since civilians toppled the 23-year presidency of strongman Zine El Abidine Ben Ali in 2011.
Armed groups have targeted Tunisian security forces and civilians in the past, including in the capital and in areas in the country’s central region and on the border with Algeria.
Earlier this month, a group allied to Isis claimed responsibility for the murder of a 16-year-old shepherd, Mabrouk Soltani, in the southern Jebel Mghila Mountains. A video posted bythe Jund al-Khilafa group claimed the teenager was an army informer.
Tunisian authorities are now building a wall along the country’s eastern border in an attempt to prevent citizens from travelling to jihadist camps in Libya. The barrier will reportedly cover about a third of the 310 mile-long border.
The European Union is expected to announce an enhanced aid programme later this year and the United States has promised to quadruple security aid to the country.