Students threw stones and petrol bombs at riot police outside university buildings late into the night after a vigil to mark the December 6 killing of 15-year-old Alexandros Grigoropoulos turned violent.

Police blocked surrounding roads and fired teargas at the youths, who sheltered in the university campus into 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.

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.