Hundreds of student protesters became trapped inside PolyU after Hong Kong police closed all university exits following violent clashes, which entailed demonstrators hurling petrol bombs, bamboo poles and bricks, and shooting arrows at police officers. Photo by Philip FONG / AFP Protesters react as police fire tear gas while they attempt to march towards Hong Kong Polytechnic University in Hung Hom district of Hong Kong on November 18, 2019.

Police warned Sunday that if "rioters" did not stop using lethal weapons and ammunition, live bullets would be fired.

At around 2 a.m. Monday morning, police stopped pro-democracy lawmakers who arrived at the scene from taking away protesters who were willing to leave campus.

They also set a fire on a footbridge connecting the campus to a railway station as well as burned trash heaps on a staircase near the university's entrance in attempts to keep police from entering the campus.

Fiery explosions were seen early Monday as Hong Kong police stormed into a university held by protesters after an all-night standoff.

The former Speaker of the House of Commons, John Bercow and former Foreign Secretary Sir Malcolm Rifkind, issued an appeal to Hong Kong's Chief Executive Carrie Lam on Sunday, urging her to order the Hong Kong Police Force to show restraint in its response to protests at Hong Kong's Polytechnic University.

Demonstrators duck behind shields as a blue-dyed liquid is sprayed from an armored police vehicle at the Hong Kong Polytechnic University in Hong Kong, Nov. 17, 2019.

Chinese state media repeatedly broadcast comments made on Thursday by Mr Xi, in which he denounced the unrest and said "stopping violence and controlling chaos while restoring order is now Hong Kong's most urgent task".

Earlier police said they were arresting students on riot related charges.

He added: "Britain in particular has not just a moral responsibility because of our history with Hong Kong but also a legal responsibility under the Sino-British Joint Declaration, to which we're a signatory".

The unrest in Hong Kong poses the gravest popular challenge to Chinese President Xi Jinping since he came to power in 2012.

The escape reportedly went undetected for more than 40 minutes, after which police began arresting those left behind. In Hong Kong "rioting" is punishable by up to 10 years in jail.

At daybreak, protesters remained in control of most of the campus.

China has repeatedly warned that it will not tolerate the dissent, and there have been concerns that Beijing could send in troops to put an end to the spiralling unrest. "We may need global help".

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.

The bill has been withdrawn, but the protests have expanded into a wider resistance movement against what is perceived as the growing control of Hong Kong by Communist China, along with calls for full democracy for the territory.

Hong Kong police routinely carry sidearms, but until now they have only used them in isolated incidents during running street clashes.

The clashes are some of the worst violence since anti-government protests began in Hong Kong five months ago.

The Hong Kong garrison "has the determination, confidence and capabilities to safeguard sovereignty, security and development interests of Hong Kong in accordance with the Basic Law and the Garrison Law and to ensure long term prosperity and stability in Hong Kong", Wu said, speaking on the sidelines of a defence conference in Bangkok.

Since June, Hong Kong has seen massive, regular demonstrations, which started in opposition to a proposed bill that would have allowed Hong Kong citizens to be extradited to the mainland.