TUNIS: Thousands of protesters backed by Tunisia's powerful labour union gathered in central Tunis on Saturday in the country's biggest demonstration for years, defying a police lockdown that blocked roads in a large area of the capital.

The rally was held to mark the anniversary of the 2013 killing of a prominent activist and to protest against police abuses that demonstrators say have imperilled freedoms won in the 2011 revolution that triggered the "Arab Spring".

Riot police deployed cordons around the city centre, stopping both cars and many people from entering the streets around Avenue Habib Bourguiba as thousands of people gathered, a Reuters witness said.

Unlike previous marches in a wave of protests that have spread across Tunisia in recent weeks, Saturday's rally was backed by the UGTT union, the country's most powerful political organisation with a million members.

Protests which began last month over inequality have increasingly focused on the large number of arrests and reports - denied by the Interior Ministry - of abuse of detainees.

Protesters chanted against the moderate Islamist Ennahda party, a member of successive government coalitions, and reprised the Arab Spring slogan: "The people want the fall of the regime".

A decade after Tunisia's revolution, its political system is mired in endless squabbling between the president, prime minister and parliament while the economy stagnates.