Deposed Tunisian President Zine al-Abidine Ben Ali 's head of security has been arrested. State media said Ali Seriati, who led Mr Ben Ali's presidential security force, is accused of threatening state security by fomenting violence. Meanwhile, there have been exchanges of fire between gunmen and security forces in the capital Tunis. Political leaders have started efforts to fill the power vacuum created by the fall of President Ben Ali. Interim leader Foued Mebazaa, who until Saturday was the speaker of parliament, has asked Prime Minister Mohammed Ghannouchi to form a national unity government. Under the present Tunisian constitution, a presidential election must be held within 60 days. Mr Ben Ali, who had been in power for 23 years, fled to Saudi Arabia on Friday after a month of mounting protests across the country over unemployment, food price rises and corruption. Dozens of people were killed as police opened fired on demonstrators. The announcement of Mr Seriati's arrest on Sunday came after the previous day saw widespread violence across Tunisia, including looting, arson and deadly jail riots. The BBC's Wyre Davies, in Tunis, reports that the sound of sporadic gunfire has been heard in and around the city centre throughout Sunday. Several reports describe a gun battle outside the headquarters of the Democratic Progressive Party. Our correspondent says there appears to be a residual base of people loyal to former regime and opposed to moves towards democratisation who are holding out against the new order. Other attacks appear to have targeted businesses and buildings connected with the former president and his family. A hospital source in Tunis told AFP news agency that Imed Trabelsi, the nephew of Mr Ben Ali's powerful wife, had been stabbed to death on Saturday. Residents in some areas have armed themselves with sticks and clubs, forming impromptu militias to protect their homes. Despite this, the interim government has shortened the overnight curfew by four hours. Curfew hours - which ran from 1700 (1600 GMT) to 0700 (0600 GMT) for two nights - will now be between 1800 and 0500. A state of emergency remains in force and there is very little economic activity. Schools, government offices and most shops are closed. Source: BBC