Hong Kong — Protesters in Hong Kong clashed with police Monday at the city's Polytechnic University. Protesters who spent days trying to keep police from getting in are now desperately trying to get out, to avoid arrest or injury.

Police have been using tear gas and water cannons.

Protesters have been using their trademark umbrellas for defense, while some threw molotov cocktails.

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It's just one of many expected flash points in the run-up to a new election in Hong Kong on Sunday, reports CBS News' Ramy Inocencio, who's there.

It wasn't clear if any of those inside the university got out.

Hong Kong's work week started with multiple protests that disrupted traffic, and schools remained closed because of safety concerns. There was a temporary lull in the pitched battles for control of the Polytechnic campus as the emphasis shifted from battering the protesters with tear gas and water cannons to waiting for them to come out.

Protesters clash with riot police as they try to leave the surrounded campus of Hong Kong Polytechnic University (PolyU), in Hong Kong on November 18, 2019 THOMAS PETER / REUTERS

For days, protesters have fortified the campus to keep police from getting in. Cornered by authorities, they were trying to emerge.

Officers repelled one attempt Monday morning with tear gas, driving a few hundred protesters back onto the campus.

The give-and-take has played out repeatedly during the city's months of anti-government unrest. The protesters want to avoid arrest. The police want to pick up as many as they can.

"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 is located.

"Other than coming out to surrender, I don't see, at the moment, there's any viable option for them," he said.

Cheuk said police have the ability and resolve to end the standoff peacefully so protesters should not "try their luck."

Derek Liu, the president of the school's student union and Owan Li, the student representative of the University Council, claimed Monday that thousands of protesters were trapped inside and police were making "massive" arrests. They told reporters 3 people had serious eye injuries and about 40 people were suffering from hypothermia after being directly hit by police water cannons. Liu and Li called on Hong Kong residents to stage a general strike to support protesters.

Protesters won on a legal front when the high court struck down a mask ban imposed by the government last month. The court said it did not consider anti-mask laws unconstitutional in general, but in this case, the law infringed on fundamental rights further than was reasonably necessary.

Many protesters wear masks to shield their identities from surveillance cameras that could be used to arrest and prosecute them. The ban has been widely ignored, and police have charged protesters with wearing masks.

Protesters attempt to leave the campus of Hong Kong Polytechnic University (PolyU) during clashes with police in Hong Kong on November 18, 2019 THOMAS PETER / REUTERS

The protests started peacefully in early June, sparked by proposed legislation that would have allowed criminal suspects to be extradited to the mainland. But by the time the bill was withdrawn, the protests had hardened and broadened into a resistance movement against the territory's government and Beijing.

Activists see the extradition bill as an example of Hong Kong's eroding autonomy under Beijing's rule since the 1997 handover from colonial power Britain.

The head of a nationalistic Chinese newspaper said Hong Kong police should use snipers to fire live ammunition at violent protesters.

"If the rioters are killed, the police should not have to bear legal responsibility," Global Times editor Hu Xijin wrote on his Weibo social media account.

Anti-government protesters barricaded themselves inside Polytechnic last week. Police surrounded the area Sunday night and began moving in after issuing an ultimatum for people to leave the area. The crowd wore raincoats and carried umbrellas to shield themselves from police water cannons.

Riot officers broke in one entrance before dawn as fires raged inside and outside the school, but they didn't appear to get very far. Fiery explosions occurred as protesters responded with gasoline bombs. Police, who have warned that everyone in the area could be charged with rioting, reportedly made a handful of arrests.

At daybreak, protesters remained in control of most of the campus. In one outdoor area, some demonstrators made gasoline bombs while others dozed while wearing gas masks. Two walked about with bows and quivers of arrows, while many stared at their smartphones.

"We are exhausted because we were up since 5 a.m. yesterday," said a protester who gave only his first name, Matthew. "We are desperate because our supplies are running low."

A lull settled on the area as the president of the university said in a video message that police have agreed to suspend their use of force.

Jin-Guang Teng said police would allow protesters to leave and he would accompany them to the police station to ensure their cases would be processed fairly.

"I hope that you will accept the proposed temporary suspension of force and leave the campus in a peaceful manner," he said.

It seemed unlikely the protesters would accept the offer given that they would all likely be arrested.

A few hundred streamed out of the campus about 8:15 a.m. in an apparent bid to escape, but they were driven back by police tear gas. Some wearing gas masks calmly picked up smoking tear gas canisters and dropped them into heavy-duty bags, but the protesters decided to retreat with a phalanx of officers lined up across the road in the distance.

Police have set up a dragnet around the campus to try to arrest protesters, who typically try to melt away after blocking traffic or causing other disruption before police run in to grab as many as they can.

Other protesters blocked a major road not far from the Polytechnic campus to distract police and help those inside the campus escape.