Thousands of protesters are gathering in central to mark the sixth anniversary of the fatal police shooting of an unarmed teenager in the Greek capital that led to widespread rioting, while clashes have broken out between demonstrators and police in the northern city of Thessaloniki.

Alexandros Grigoropoulos, 15, had been out with friends in a central Athens neighborhood when he was shot on Dec. 6, 2008 following a verbal altercation with police. His death led to two weeks of the most violent rioting Greece had seen in decades.

This year's anniversary marches come as nearly nightly violent protests by supporters of one of Grigoropoulos' friends, jailed anarchist and convicted bank robber Nikos Romanos. He is on hunger strike demanding prison leave to attend lectures after he passed university entrance exams.