Young protesters have set the police headquarters in the Tunisian town of Kef on fire and tried to break open the local prison, local trade unionists say.
Several police cars were set alight and the fire threatened to spread to neighbouring buildings, sources told the AFP news agency.
The army prevented the youths from freeing prisoners in the local jail and a military helicopter circled over the town Sunday afternoon.
The riot erupted the day after four people were killed and about 15 wounded when police opened fire on demonstrators in the town.
That led to attacks on a police station and the headquarters of the RCD party of deposed President Zine El Abidine Ben Ali.
The crowds were demanding the sacking of the local police chief, whom they accused of abuses of power.
A young man has been killed by a police tear gas canister in the southern town of Kebili, according to the official Tunisian news agency, TAP.