Saturday's unrest followed a memorial gathering at 21:00 local time [19:00 GMT] at the site where Alexandros Grigoropoulos was killed on December 6 in Athens' Exarchia district.
Police blocked surrounding roads and fired teargas at the youths, who sheltered in the university campus which police are banned from entering. A group of anxious mothers waited outside to escort their children from the building.
"There are more than 600 students and they're running in and out of the university, throwing rocks and Molotov cocktails," said a police official, who asked not to be named. No injuries were reported.
Across the country, hundreds of schools and several university campuses remain occupied by students. In the northern city of Thessaloniki, demonstrators briefly occupied a radio station and a cinema.
The protests, the worst Greece has known in decades, have fed on anger at youth unemployment, government reforms and the global economic crisis.
For most of Saturday, Athens was calm and the streets were busy with Christmas shoppers.
Earlier, a march in support of immigrants' rights ended in scuffles with police when demonstrators pelted them with eggs and rubbish outside parliament.
Some protesters tried to set fire to the municipal Christmas tree in the central Syntagma square outside parliament, a replacement for a tree burnt down in earlier demonstrations. Riot police with shields formed a circle round the tree while protesters danced round them holding hands.
Union leaders and students have announced more rallies for the new year.
The protests have caused hundreds of millions of euros in damage, rocking a conservative government that has a one-seat majority and trails the opposition in polls.
Some analysts say continued street protests could force early elections.
Last Mod: 21 Aralık 2008, 13:01