Tunisia's President Beji Caid Essebsi declared a nationwide state of emergency and a curfew in the capital after a bomb attack on a presidential guard bus killed at least 12 people.

A security source at the site said "most of the agents who were on the bus are dead" after the attack in Tunis, which has become a target of jihadist violence since the 2011 revolution.

No group immediately claimed responsibility for the bombing, which a ministry official said also wounded 20 people when it went off on Mohamed V Avenue, just as this year's 26th Carthage Film Festival was in full swing.

Essebsi, who cancelled a trip to Switzerland for Wednesday, declared a state of emergency throughout the country and a curfew in the capital.

"As a result of this painful event, this great tragedy... I proclaim a state of emergency for 30 days under the terms of law and a curfew in greater Tunis from 9:00 pm (2000 GMT) until 5:00 am tomorrow," he said in brief televised address.

Presidential spokesman Moez Sinaoui, who described the bombing as an "attack", told AFP the curfew would stay in place until further notice.

An AFP journalist on the scene saw the partly burnt-out shell of the bus, along with police, ambulances and fire trucks. Many people were in tears.

A bank employee working nearby reported hearing a large explosion and seeing the bus on fire.

Carthage Film Festival director Brahim Letaief cancelled the night's screenings, saying he hoped the showcase for African and Arab film-makers could resume on Wednesday.

"That is the only way to respond to these barbaric acts," he told AFP.

Prime Minister Habib Essid and Interior Minister Najem Gharsalli visited the scene of the blast.

- IS attacks on foreigners -

The United States condemned the attack and offered to help the Tunisian authorities with their investigation.

US Secretary of State John Kerry "was proud to stand with Tunisian leaders earlier this month in Tunis and reaffirm our countries' extensive economic, governance, and security cooperation", a spokesman for his department said.

The UN Security Council urged its members to help bring the perpetrators to justice, adding that "no terrorist attack can reverse the path of Tunisia towards democracy and its efforts towards economic recovery".

Tunisia has been plagued by Islamist violence since the 2011 overthrow of longtime dictator Zine El Abidine Ben Ali, and dozens of members of the security forces have also been killed.

Two attacks 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.

A state of emergency was imposed after the Sousse killings and later renewed before being lifted at the beginning of October.

On Sunday, a jihadist group claimed the beheading of a young Tunisian shepherd on behalf of IS, accusing him of having informed the army about their movements in the central province of Sidi Bouzid.

The killing of 16-year-old Mabrouk Soltani on November 13 sparked anger in Tunisia. His killers ordered a 14-year-old who was working with him to bring the victim's head wrapped in plastic to his family.

The video in which the claim was made, whose authenticity could not be confirmed, said the young shepherd gave information on "the soldiers of the Islamic State" to the Tunisian army. That was denied by the government.

Seven women were recently detained for engaging in pro-IS propaganda, while 20 people were arrested on suspicion of planning attacks on hotels and security facilities.

Thousands of Tunisia citizens are fighting in neighbouring Libya, as well as in Iraq and Syria on the side of jihadists.