Police fanned out throughout central Tunis after Tuesday’s explosion, and ambulances rushed to the scene, evacuating wounded and dead. Prime Minister Habib Essid and Interior Minister Najem Gharsalli visited the scene of the attack after it was cordoned off by police.
Witness Bassem Trifi, a human rights lawyer, said the explosion hit the driver's side of the bus, describing a “catastrophic spectacle.”
"I saw at least five corpses on the ground," he told The Associated Press. "This was not an ordinary explosion."
Emir Sfaxi, another witness and activist, told Al Jazeera “there is a lot of confusion” and that city residents are “scared.”
“There are traffic jams as everyone is trying to get home quickly,” he added.
The attack comes 10 days after authorities increased the security level in the capital and deployed security forces in unusually high numbers.
Earlier this month, Tunisian authorities announced the dismantling of a cell it said had planned attacks at police stations and hotels in the seaside city of Sousse, about 95 miles southeast of Tunis.
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 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 Islamist 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 wire services
Error
Sorry, your comment was not saved due to a technical problem. Please try again later or using a different browser.