The trouble came on the day that members of Tunisia's interim government formally resigned, following the formation of the country's first-ever democratically elected constituent assembly.
Video posted on YouTube purports to be taken during clashes in Kasserine: SBS cannot verify its accuracy.
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The flashpoint was in Kasserine, 350 kilometres (220 miles) from Tunis, where thousands of residents protested that the "martyrs" from the town who died in the revolution which allowed the democratic moves to take place, were being forgotten, trade union leader Sadok Mahmudi told AFP.
Sixty-six people were admitted to hospital suffering from the teargas, a medical source said.
Mahmudi said the demonstrators had hurled stones at the security forces and attempted to prise open the gates of a local prison.
After the trouble, military reinforcements were brought in and sealed off the area.
The interior ministry confirmed the incident at Kasserine and said there was also some trouble at Thala and Feriana in the same area.
The transitional government headed by Beji Caid Essebsi had been in charge in Tunisia since the end of February, six weeks after a popular uprising forced longtime dictator Zine el Abidine Ben Ali into exile.
Tunisia's three main parties formalised a power-sharing agreement on Monday following October 23 elections for the constituent assembly.
The 217-member body will be tasked with drafting a new constitution and picking a new executive, until when Essebsi's team will continue to run government affairs.
Last month's vote was dominated by the Islamist Ennahda party, whose Hamadi Jebali is due to be prime minister under the power-sharing deal.
