Hong Kong — Police laid siege to a university on Monday, firing rubber bullets and tear gas to pin back antigovernment protesters armed with petrol bombs and other weapons to stop them from fleeing amid fears of a bloody crackdown.

Dozens, choking on the tear gas, tried to leave the Polytechnic University by breaking through police lines after a night of mayhem in the Chinese-ruled city in which roads were blocked, a bridge set on fire and a police officer was shot by a bow and arrow...