The students threw fire bombs at police lined up to guard their headquarters and officers responded by briefly firing small amounts of tear gas, the Voice of America reported.
Smaller groups of youths, meanwhile, threw eggs at police guarding Athens' main courthouse while other students said they would stage a march on Parliament later in the day.
Witnesses told VOA that the intensity of Monday's protests has tailed off sharply, and came after a day of relative calm on Sunday.
The riots were sparked by the Dec. 6 police shooting death of a teenage boy, quickly became violent and spread to other Greek cities. Protest leaders told VOA Sunday they are decrying a widening gulf between rich and poor in Greece and are protesting high-level corruption.