TAREK AMARA THOUSANDS of protesters backed by Tunisia's powerful labour union gathered in central Tunis yesterday 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…