Police fired warning shots and tear gas Thursday to disperse protesters who attacked provincial government headquarters in the town where Tunisia's revolution was born, an AFP correspondent witnessed.
Demonstrators also tried to torch the local headquarters of the ruling Islamist party Ennahda, after some of them broke down the door and sacked the offices.
Dozens of people, angry over their living conditions, converged on the government building in Sidi Bouzid and set fire to a tyre, which they threw inside.
Police responded with warning shots and tear gas, as demonstrators shouted "Ben Ali's police are back," in reference to the long-time dictator driven from office last year by a popular revolt.
Calm was eventually restored, the interior ministry said, and no was one hurt.
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The uprising against Zine El Abidine Ben Ali touched off a wave of political unrest across North Africa and the Middle East dubbed the Arab Spring. It was sparked when Sidi Bouzid street vendor Mohamed Bouazizi immolated himself in protest over his own precarious livelihood.
The interior ministry confirmed that the violence had occurred, but spokesman Khaled Tarouch denied that warning shots had been fired.
"Police only fired tear-gas grenades to disperse the people who had attacked the government headquarters with rocks," he said.
The ministry said about 150 people, day workers demanding to be paid, were involved. Union sources said more than 1,000 people took part.
The workers, who had not received their wages in several weeks, had been protesting peacefully for a number of days until being egged on by relatives and residents.
An Ennahda spokesman deplored the sacking of the party's offices by what he called "a group of demonstrators manipulated by political parties," which he did not name.