Around 100 others who tried to leave the Polytechnic University were met with tear gas and rubber bullets. Some were arrested.
Authorities say 116 people were injured in the violence on Monday.
In the past week, the campus has become the latest battleground for long-running anti-government protests.
The violence is some of the worst seen during months of unrest in the semi-autonomous Chinese territory. The protests started over a controversial extradition bill, and have now evolved into broader anti-government demonstrations.