TUNIS — At least 12 people were killed on Tuesday after an explosion tore through a bus full of Tunisian presidential guards in an attack one source said was probably carried out by a bomber detonating his explosives in the vehicle.

Ambulances rushed wounded from the scene and security forces closed off streets around Mohamed V Avenue, one of the major streets in the capital Tunis and where the charred wreckage of the bus lay not far from the interior ministry.

Tunisian police block the road leading to the site of an explosion on a bus transporting Tunisia's presidential guard in central Tunis on November 24. FETHI BELAID / AFP - Getty Images

It is the third major attack to strike Tunisia this year after a militant killed 38 foreigners at a beach hotel in June and gunmen killed 21 tourists at the Bardo Museum in Tunis in March. Islamic State claimed both those attacks.

Security sources said the guards were boarding the bus to be taken to the presidential place on the outskirts of the city when the explosion hit. One presidential source said it was likely that a bomber had detonated his explosive belt inside.

"I was on Mohamed V, just getting ready to get into my car, when there was a huge explosion. I saw the bus blow up. There were bodies and blood everywhere," said Bassem Trifi, a witness.

At least 12 guards were killed and 17 wounded, according to an interior ministry statement.

President Beji Caid Essebsi canceled a trip to Europe he had planned for Wednesday.

Mohamed V is a major boulevard usually packed with traffic and pedestrians, and the site of several hotels and banks.

Fighting Islamist militants 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 Islamic State and other militant groups, and some have threatened to carry out attacks at home.

The army has also been fighting against another Islamist militant group in the mountains near the Algerian border. Militants have hit checkpoints and patrols in rural areas in the past.