Hong Kong police laid siege to a university on Monday, firing rubber bullets and tear gas to keep back anti-government protesters armed with petrol bombs and other weapons and 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.

Dressed in regular clothing and without gas masks, the protesters, tried to exit, dodging tear gas canisters and sponge grenades, only to be forced back inside.

Some were arrested, tackled to the ground, as others scrambled and tripped over barricades and fences as police pointed guns at them and threw punches.

“The police might not storm the campus but it seems like they are trying to catch people as they attempt to run,” Democratic lawmaker Hui Chi-fung told Reuters.

“It’s not optimistic now. They might all be arrested on campus. Lawmakers and school management are trying to liaise with the police but failed.”

Police said officers had been deployed “on the periphery” of the campus for a week, appealing to “rioters” to leave.

“All our warnings were ignored,” they said in a statement. “Our message was loud and clear, the violence has escalated to rioting.”

They said “toxic and dangerous chemicals” had been stolen from the university laboratory.

“We must warn that the university campus has become a powder keg where danger is far beyond what we can estimate,” police said.

So far, 154 people between the ages of 13 and 54, were arrested over the weekend, the police said.

Dan, a 19-year-old on the campus, burst into tears as he said the protesters had been trapped for too long.

“We need all Hong Kongers to know we need help,” he said. “I don’t know how much longer we can go on like this. We may need international help.”

One 24-year-old protester, who gave his name as “Be Patient”, said he nearly suffocated in the crush.

“We couldn’t move at all. The police didn’t stop … they still used rubber bullet and sponge rounds to attack us. We’re talking about a distance of one metre.”

There were also running battles in the nearby commercial area of Nathan Road where activists stopped traffic and forced shopping malls and stores to shut.

Police said a car tried to hit an officer in the nearby Yau Ma Tei district on Sunday night, reversing and trying again. Police fired a shot and the driver fled.

Thirty-eight people were wounded overnight on Sunday, the Hospital Authority said.

Reuters witnesses saw some protesters suffering from burns from chemicals in jets fired from police water cannons.

Police said they fired three live rounds when “rioters” attacked two officers who were attempting to arrest a woman. No one was wounded and the woman escaped amid a dramatic escalation of the unrest that has plunged the Asian financial hub into chaos for almost six months.

Demonstrators are angry at what they see as Chinese meddling in Hong Kong’s promised freedoms when the then British colony returned to Chinese rule in 1997. They say they are responding to excessive use of force by police.

China says it is committed to the “one country, two systems” formula granting Hong Kong autonomy, while the city’s police deny accusations of brutality and say they show utmost restraint.

China’s foreign ministry said on Monday no one should underestimate its will to protect its sovereignty.

“If you are viewing the facts with an impartial view you can see that what is happening in Hong Kong is no longer a simple demonstration, it is a handful of violent criminals conducting violence against civilians,” ministry spokesman Geng Shuang told a briefing.

Chinese soldiers in a base close to the university were seen on Sunday monitoring developments at the university with binoculars, some dressed in riot gear.

Chinese troops in shorts and T-shirts, some carrying red plastic buckets or brooms, emerged from their barracks on Saturday in a rare public appearance to help clean up debris.

Chinese troops have appeared on Hong Kong’s streets only once since 1997, to help clear up after a typhoon last year.

The unrest poses the gravest popular challenge to Chinese President Xi Jinping since he came to power in 2012. Beijing denies interfering in Hong Kong’s affairs and has blamed Western countries for stirring up unrest.

The Hong Kong government invoked a colonial-era emergency law in October banning faced masks commonly used by protesters. The High Court ruled on Monday the ban was unconstitutional and police said they would suspend all such prosecutions.