TUNIS: Tunisia's government Tuesday condemned as "terrorism" a spate of overnight attacks on courts and other state buildings by gangs including Islamist hardliners and vowed to punish them.

The ultra-conservative Salafists denied involvement in the rampage in several areas of the capital Tunis and in the country's northwest, and instead called a protest after this week's Friday prayers.

The violence has fuelled fears among moderate Tunisians over the rise of the radicals since a revolution last year toppled the Zine El Abidine Ben Ali regime and elections ushered in a moderate Islamist government.

The attacks, which led to 90 arrests, raised questions over who masterminded the violence and renewed doubts over whether the moderate Islamist government will be able to stamp it out.

Police fired tear gas to quell attacks by Salafists and others who torched and pillaged a local court in west Tunis and attacked several police stations in the north, ministry spokesman Khaled Tarrouche said.

In the northern suburb of La Marsa, rioters tried to force their way into an art gallery where several paintings deemed "blasphemous" had been slashed a day earlier by Islamists. Seven officers were slightly injured.

Justice Minister Nourredine Bhiri condemned the "terrorist act" and pledged that the guilty would "pay a heavy price."

"These are terrorist groups which have lost control, they are isolated in society," Bhiri told radio Shems FM.

"We arrested 90 people from the Salafist and criminal circles," said another Interior Ministry spokesman, Lotfi Hidouri.

At the same time, in what appeared to be a concession to the Salafists, the ruling Ennada party announced that it would move to include in the constitution a provision against blasphemy.

"Religious symbols are above all derision, irony or violation," said the party, adding that freedom of expression is "not absolute and those who exercise it must respect the beliefs and customs of the people."

The Salafist movement said it had nothing to do with the overnight violence, and called instead on Tunisians to protest against "infringements against Islam."

"Ansar Al Charis calls on all Tunisians to demonstrate across the country after Friday prayers to protest against religious infringements," said Sami Essid, speaking on behalf of the Islamist group's chief.

"We have nothing to do with what happened yesterday in Tunis."

Another radical imam, Abou Ayoub, said in a video circulated on Facebook: "The Muslim population must rise up Friday after prayers in response to those who mock Islam."

"Since the fall of Ben Ali, the infidels have not stopped mocking our religion, and it's becoming more frequent every day," said Ayoub who had called in October 2011 for attacks against television station Nessma after it broadcasted the French-Iranian animated film "Persepolis".

The Salafists have in recent weeks intensified their violence in Tunisia, attacking police stations and bars. Some critics have asked if the government has been too lax in dealing with radicals.

Monday night's events sparked questions on who organised the rioting.

Commentators said unrest started just two days after Al-Qaeda chief Ayman al-Zawahiri called on Tunisians to rise up to claim sharia law.

Others said they wondered whether it was a plot by Ben Ali sympathisers to destabilise the country and reclaim power.

"The fact that the violence erupted in several places at the same time makes us think that it was organised," said Tarrouche.

At Essijoumi in west Tunis, the court prosecutor's office was burnt down. Other areas in the west of the city, were also hit.

The regional offices of the powerful union UGTT in Tunisia's northwestern town of Jendouba were torched early Tuesday and attackers also set fire to a truck that was transporting alcohol in the town, TAP agency said.

Several people arrested said they "were paid by Salafists to carry out the acts of destruction," security sources quoted by TAP said.

The Salafist movement comprises several branches. Some adherents focus strictly on religion, some are politicians and others are jihadists who see violence as a legitimate means to impose their faith.

Researchers put Salafist numbers in Tunisia at around 10,000. -- AGENCIES