The Islamic State in Iraq and the Levant (ISIL) claimed responsibility for Tuesday's attack on a Tunisian presidential guard bus, a statement from the group said on Wednesday.
Tunisian authorities said that the suicide attack killed at least 12 guards and led the government to impose a nationwide state of emergency. Another body at the scene is believed to be of the attacker, who ISIL identified as Abu Abdullah al-Tunisi.
An Interior Ministry statement said the blast of 22 pounds of explosives was located either in the bomber's backpack or a belt he was wearing.
The explosion on a main boulevard in the capital drove home the vulnerability of Tunisia to attacks, following gun assaults on a seaside tourist hotel in June and the Bardo Museum in Tunis in March, both claimed by ISIL.
Around Tunis on Wednesday, troops and armed police patrolled the city streets and set up checkpoints searching vehicles and pedestrians. At Tunis international airport security forces were allowing in only people with booked flights.
Security officials said the bomber blew himself up as presidential guards were boarding a bus on Mohamed V Avenue to travel to the presidential palace for duty.
Fighting armed groups has become a major challenge for Tunisia, the small North African country that was hailed as a blueprint for democratic change in the region after an uprising in 2011 ousted longtime autocrat Zine Abidine Ben Ali.
Tunisia has had free elections and is operating under a new constitution and a broad political consensus that has allowed secular and religious parties to overcome a crisis that threatens to overturn their young democracy.
But several thousand Tunisians have also left to fight in Syria, Iraq and Libya with ISIL and other armed groups, and some have threatened to carry out attacks at home.
A luxury beach hotel in Sousse was hit by an attack last June that left 38 people dead. In March, an attack at Tunisia's famed Bardo museum near the capital killed 22 people. ISIL claimed responsibility for both attacks.
The army has also been fighting against another armed group in the mountains near the Algerian border. Rebels have hit checkpoints and patrols in rural areas in the past.
Al Jazeera and Reuters