Violence over shooting of teenager continues as unions ground flights and shut offices.
The rioters periodically retreat to within the university building when overwhelmed by tear gas used by anti-riot police to subdue the protesters.
Security forces are prevented from entering the building by law, although they can gain access if the university administration grants permission, something they have so far declined to do.
Greek cities including Athens and Thessaloniki have been hit by a wave of protests and rioting since Grigoropoulos’ shooting by police officers.
While some anti-government protests had already been planned by left-wing opposition groups angered by a series of corruption scandals that have seen three cabinet ministers forced from office, demonstrations turned violent following the shooting.
While two officers having been arrested and charged in relation to Grigoropoulos’ death, demonstrators are arguing for the police to be disarmed to prevent what they see as random police violence.
Earlier on Saturday, around 150 youths attacked the Christmas tree at Syntagma Square in central Athens, tossing garbage and hanging rubbish bags from its branches before clashing with anti-riot police.
In Thessalonki, demonstrators occupied a cinema theatre and hurled sweets and cakes at the city’s mayor, Vassilis Papageorgopoulos, who was attending a charity event nearby.
The latest round of violent clashes will have dashed police and government hopes that the protests would dwindle as the Christmas season got underway.