Officials say there was no immediate claim of responsibility for the attack against the presidential guard.
Walid Louguini, spokesman for the Tunisian interior ministry, told the Associated Press that at least 12 people were killed
Louguini also affirmed that no fewer than 20 others were wounded in the attack.
A witness, Bassem Trifi, said the explosion hit the driver’s side of the bus. He described the scene as “catastrophic”.
A few days earlier, the authorities had increased the security level in the capital and deployed security forces in large numbers.
Mark Toner, the US state department spokesman speaking in Washington, said the US government was still seeking details on what had happened in Tunis.
Toner further added: “We strongly condemn the attack.”
Following the explosion, President Beji Caid Essebsi, who was not on the bus at the time, declared a 30-day state of emergency across the country and imposed an overnight curfew for the capital.
He convened an emergency meeting of his security council for Wednesday, November 25.
Speaking on national television, Essebsi said: “Tunisia is at war against terrorism.”He urged international cooperation against extremists who have killed hundreds around Europe and the middle-east in recent weeks, from Paris to Beirut to a Russian plane shot down over Egypt.
“I want to reassure the Tunisian people that we will vanquish terrorism,” he added.
Tunisia is the only Arab Spring country to have solidified a new democracy. However, the nation is facing serious economic and security challenges.
In June, shootings at a luxury beach hotel in Sousse, left 38 people dead, mostly tourists.
Also in March, an attack by Islamist extremists at the famed Bardo museum near the capital killed 22 people.
The two acts of terrorism this year have left a great dent in Tunisia’s tourism industry.