Riots raged for hours in central Athens Tuesday as dozens of anarchists threw petrol bombs, fireworks and stones at police after a peaceful march to commemorate the police killing of an unarmed teenager eight years ago.

Riot police used tear gas to contain young people who set up barricades around a university complex in the Exarcheia residential district, a traditional venue for protests that often turn violent.

Other rioters pelted police with the stones and petrol bombs from the roofs of surrounding apartment buildings. Police said one officer was hospitalized with light injuries, and at least 23 people were detained.

Young demonstrators also used petrol bombs and rocks to attack police during a separate clash in northern Greek city of Thessaloniki.

About 1,000 people had protested in the city center earlier Tuesday to mark the 2008 fatal shooting of 15-year-old Alexandros Grigoropoulos.

In Athens, about 1,500 anarchists and other demonstrators marched peacefully through the city to protest Grigoropoulos' killing by a police officer in the Exarcheia district, which sparked days of rioting in Greece's main cities.

Anarchists initiated violent clashes with police in Exarcheia last month, both during a visit by President Barack Obama and on the anniversary of a 1973 student uprising.