A security source at the site of the attack said "most of the agents who were on the bus are dead".
The explosion, described as a "attack" by presidential spokesman Moez Sinaoui, struck on the capital's Mohamed V Avenue, a ministry official told AFP.
Twenty people were wounded.
President Beji Caid Essebsi, who cancelled a trip to Switzerland for Wednesday, declared a state of emergency throughout the country and curfew in the capital.
The bombing, which has still not been claimed, took place as this year's 26th Carthage Film Festival was in full swing.
Festival director Brahim Letaief had already cancelled the night's screenings, saying he hoped the showcase for African and Arab film-makers could resume on Wednesday.
"That is the only way to respond to these barbaric acts," he told AFP.
An AFP journalist reported seeing the partly burnt-out shell of the bus, with police, ambulances and fire trucks at the scene.
Many people were in tears.
A bank employee working nearby reported hearing a large explosion and seeing the bus on fire.
While there was no immediate claim of responsibility, Tunisia has been plagued by Islamist violence since the 2011 overthrow of longtime dictator Zine El Abidine Ben Ali, and dozens of members of the security forces have also been killed.
Two attacks this year claimed by the Islamic State group targeted foreigners -- at the National Bardo Museum in March, killing 21 tourists and a policeman, and at a resort hotel in Sousse in June, killing 38 tourists.
The killing of 16-year-old Mabrouk Soltani on November 13 sparked anger in Tunisia. His killers ordered a 14-year-old who was working with him to bring the victim's head wrapped in plastic to his family.
The video in which the claim was made, whose authenticity could not be confirmed, claimed the young shepherd gave information on "the soldiers of the Islamic State" to the Tunisian army.
You’ve reached your limit of {{free_limit}} free articles this month.
Subscribe now for unlimited access.
Already subscribed? Log in
Subscribe to read the full story →
Smart Quarterly
₹900
3 Months
₹300/Month
Smart Essential
₹2,700
1 Year
₹225/Month
Super Saver
₹3,900
2 Years
₹162/Month
Renews automatically, cancel anytime
Here’s what’s included in our digital subscription plans
Exclusive premium stories online
Over 30 premium stories daily, handpicked by our editors


Complimentary Access to The New York Times
News, Games, Cooking, Audio, Wirecutter & The Athletic
Business Standard Epaper
Digital replica of our daily newspaper — with options to read, save, and share


Curated Newsletters
Insights on markets, finance, politics, tech, and more delivered to your inbox
Market Analysis & Investment Insights
In-depth market analysis & insights with access to The Smart Investor


Archives
Repository of articles and publications dating back to 1997
Ad-free Reading
Uninterrupted reading experience with no advertisements


Seamless Access Across All Devices
Access Business Standard across devices — mobile, tablet, or PC, via web or app
