TUNIS (Reuters) - Tunisian police fired into the air to disperse rioters in the capital early on Sunday, and were attacked by crowds throwing petrol bombs in another city, in the most violent clashes yet. The rioting is the starkest sign to date of the friction between Tunisia's govt and Islamists who have been growing more assertive since the country's autocratic leader was ousted in a revolution six months ago. Sunday's violence was sparked by an incident on Friday when police, trying to break up an anti-govt demonstration in the center of Tunis, fired tear gas inside a mosque.