HONG KONG (AP) — Police tightened their siege of a university campus where hundreds of protesters remained trapped overnight Tuesday in the latest dramatic episode in months of protests against growing Chinese control over the semi-autonomous city.

In yet another escalation for the movement, protests raged across other parts of the city, fueled by palpable public anger over the police blockade of Hong Kong Polytechnic University and the desire to help the students stuck inside.

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 off of two campuses in the past week.

China, which took control of the former British colony in 1997 promising to let it retain its autonomy, flexed its muscles, sending troops outside their barracks over the weekend in a cleanup operation.

China’s ambassador to Britain accused the U.K. and the U.S. of meddling in the country’s internal affairs and warned that the Chinese government “will not sit on our hands” if the situation in Hong Kong “becomes uncontrollable.”

“These rioters, they are also criminals. They have to face the consequences of their acts,” said Cheuk Hau-yip, the commander of Kowloon West district, where Polytechnic University is located.

“Other than coming out to surrender, I don’t see that there’s any viable option for them,” he said, adding that police have the ability and resolve to end the standoff.

Authorities, meanwhile, were dealt a setback Monday when Hong Kong’s high court struck down a contentious ban on wearing face masks in public imposed last month, ruling it unconstitutional.

The pitched battle for control of Hong Kong Polytechnic University began last week as demonstrators for days fortified the campus to keep the police out. On Monday, cornered by security forces determined to arrest them, they desperately tried to get out but faced a cordon of officers armed with tear gas and water cannons.

Senior government officials said they were trying to de-escalate the situation and urged the protesters to leave peacefully and cooperate with police — advice that seemed certain to lead to arrests and therefore strengthened the protesters’ resolve to resist.