A bomb has exploded on a bus transporting Tunisia’s presidential guard in central Tunis, killing at least 12 people and injuring 16, the interior ministry said.

A security source at the site of the explosion said “most of the agents who were on the bus are dead.”

The explosion, described as an “attack” by presidential spokesman Moez Sinaoui, struck on the capital’s Mohamed V Avenue, a ministry official said.

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A presidential source said the attack was likely caused by a bomber detonating explosives inside the vehicle.

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.

Two attacks earlier 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.

Militants have hit checkpoints and patrols in rural areas in the past.