The confrontation happened in the central Athens neighborhood of Exarchia where 15-year-old Alexandros Grigoropolous was killed by a police officer Dec. 6. Groups of demonstrators threw rocks and petrol bombs at police, who responded with volleys of tear gas, the BBC reported.

The student protesters have established the National Technical University of Athens as their base and school administrators are barring police from entering the facility. The students are calling for action against high youth unemployment and protesting unpopular government reforms.

Another group of youths Sunday attacked the Christmas tree in Athens' Syntagma Square, hurling garbage at it and attempting to set it on fire. Riot police ringed the tree, which is a replacement for one burnt down at the height of the rioting on Dec. 8, the BBC reported.