Tunisia has declared an overnight curfew on eight regions, including the capital, an interior ministry official says, in the wake of serious rioting.The eight-hour curfew was introduced after several violent attacks in protest at an art exhibition.The government has blamed ultra-conservative Islamists known as Salafists for the violence, in which police stations were set on fire.But the Salafists have denied being involved in the rampage.The curfew will be in place in the suburbs of Ben Arouss, Ariana and Manouba as well as the cities of Sousse, Monastir, Jendouba and Ben Guerdane.'Heavy price'Monday's overnight series of attacks targeted police stations, a court and an art gallery.In one northern suburb, 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.The exhibition included paintings that caricatured Mecca, portrayed a nude woman, and showed the word "Allah" spelled out with strings of ants.The offices of Tunisia's main labour union in the north-west city of Jendouba were also set alight by Salafis overnight, state television reported according to Reuters.The country's Justice Minister Nourredine Bhiri said that those behind the violence would "pay a heavy price"."These are terrorist groups which have lost control, they are isolated in society," he told a Tunisian radio station.He said some of the violence, which came hours after al-Qaeda leader Ayman al-Zawahiri urged Tunisians to turn against the government, may have been inspired by the group.An official from the justice ministry said that those arrested would be brought to justice under a 2003 anti-terror law, AFP reports.The Salafist movement called on Tunisians to protest against "infringements against Islam" but said it had nothing to do with the violence.Since the fall of Tunisia's autocratic leader Zine al-Abidine Ben Ali, in January 2011, there has been a resurgence of hard-line Islamists in the country.Some of the Salafists' most radical branches have been holding demonstrations to demand an introduction of Sharia in Tunisia.