A police officer fires tear gas to break up a protest in the city of Kairouan.

Kairouan, Tunisia - Supporters of the hardline Islamist group Ansar al-Sharia clashed with Tunisian police on Sunday after the government banned its annual rally, saying it posed a threat to society.

Ansar al-Sharia, which openly supports al Qaeda, is considered the most radical of the hardline Islamist groups to emerge in Tunisia since a 2011 revolution overthrew secular dictator Zine al-Abidine Ben Ali.

The annual rally, expected to have drawn tens of thousands of members, was due to have been held in the central city of Kairouan, and supporters there threw stones at police, who fired teargas in response, a Reuters witness said.

Police also prevented the group holding a smaller religious meeting in the Ettadamen district of Tunis on Sunday, prompting clashes with the Salafists, who chanted: "The rule of the tyrant should fall", another Reuters witness said.

Police there fired teargas and shots into the air and to disperse about 500 protesters throwing stones at officers.

Military aircraft were patrolling the skies over the district.

Ansar al-Sharia said police had arrested its spokesman Saifeddine Rais. It was not immediately clear where or when he had been arrested, but a security source confirmed he had been detained.

The Interior Ministry said on Friday it had banned the gathering of the group, "which has shown distain for state institutions, incited violence against them and poses a threat to public security".

Hardline Islamist Salafists are seeking a broader role for religion in Tunisia, alarming the secular elite which fears their agenda is to impose strict views on people and compromise individual freedom, women's rights and democracy.

Tunisian police blamed a Salafist for the assassination of secular opposition politician Chokri Belaid on February, which provoked the biggest street protests in Tunisia since the overthrow of Ben Ali. - Reuters