It's not clear how many students remained inside the building, which now more closely resembles a warzone than a school campus.
A Reuters journalist at the scene saw dozens of students voluntarily leave or attempt to escape.
Some of those who fled climbed down plastic hosing from a bridge and climbed on waiting motorbikes as the police fired projectiles.
Many appeared to be captured shortly after.
The university is at the center of a standoff in the past week that has seen the most intense violence in five months of anti-government demonstrations.