Groups of protesters have been trying to leave the Polytechnic University of Hong Kong (PolyU), but have been met with tear gas and rubber bullets fired by police surrounding the campus.
Some protesters fought back with petrol bombs and bricks before retreating.
University officials had said earlier on Monday that police would not use force and let protesters leave peacefully, if protesters themselves did not use force.
Police later said they fired tear gas as they were faced with “rioters suddenly charging at them”.