Police fired tear gas Monday to disperse a rally on a central Tunis avenue where demonstrations are banned.
Hundreds of demonstrators, marking Martyrs' Day and protesting against the ban imposed on 28 March, sought shelter in neighbouring streets and shops.
Wrapped in Tunisian flags and shouting "We're not afraid, the people are here", the demonstrators had begun running up Habib Bourguiba Avenue around 10:00am (0800 GMT), defying police stationed along the main thoroughfare with helmets and batons.
"I'm here to honour our martyrs, and to protest against the ban on demonstrating here. We're the ones who freed Tunisia, they don't have the right to ban our peaceful marches," septuagenarian protester Mohsen Ben Henda told AFP.
Other demonstrators said they were protesting against governing Islamist party Ennahda.
"We came here today to demand our freedoms, to denounce the repression that Ennahda militias inflict on us every day," said Raed Korbi, a young doctor who had taken refuge in a cafe.
Martyrs' Day commemorates the bloody crackdown by French troops on a protest in Tunis on 9 April, 1938.
Tunisia's interior minister banned demonstrations on Bourguiba Avenue, a symbol of the Tunisian revolution and a common site for rallies, shortly after Islamist protesters demanding sharia law last month attacked actors who had gathered for a separate rally for World Theatre Day.
Saturday, police forcefully dispersed a protest by thousands of unemployed graduates who tried to march on the avenue, wounding about 20, according to organisers.