City leader Carrie Lam said 600 people had left the Hong Kong Polytechnic campus, including 200 who are under 18 years old.
Police have surrounded the university and are arresting anyone who leaves. Lam said those under 18 would not be immediately arrested but could face charges later.
She said that the other 400 who have left have been arrested.
Now in its fifth month, the Hong Kong protest movement has steadily intensified as local and Beijing authorities harden their positions and refuse to make concessions.
Universities have become the latest battleground for the protesters, who used gasoline bombs and bows and arrows in their fight to keep riot police backed by armored cars and water cannon out of two campuses in the past week.