Hong Kong police on Monday laid siege to a university campus where hundreds of anti-government protesters were holed up — firing rubber bullets and tear gas under a barrage of bricks and Molotov cocktails.

Fiery explosions made Polytechnic University look like a war zone, with a footbridge engulfed in flames as the two sides clashed in some of the most dramatic scenes since protests started in June.

Many demonstrators — carrying umbrellas to avoid water cannons — were arrested, tackled to the ground as police pointed guns at them and threw punches to stop them from fleeing.

It was the culmination of growing clashes at the campus, with a police officer shot in the leg with an arrow on Sunday.

The dramatic escalation in force raised fears of a bloody crackdown in the semi-autonomous Chinese territory, sparking criticism from the US.

“We condemn the unjustified use of force and urge all sides to refrain from violence and engage in constructive dialogue,” a senior official in President Trump’s administration told Reuters on Sunday.

Britain raised similar concerns Monday.

“We remain seriously concerned by the situation in Hong Kong and the escalation of violence between protesters and police,” said a spokesman for Prime Minister Boris Johnson. “We continue to urge for calm and restraint on all sides and support the right to peaceful protest. We believe political dialogue is the only way forward and we want to see the Hong Kong authorities agree to a path to resolve the situation.”

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.

Police arrested 154 people between the ages of 13 and 54 over the weekend.

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

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.

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.

Amid the escalation of violence, protesters celebrated a legal victory as Hong Kong’s High Court ruled Monday that a British colonial-era emergency law revived by the government to ban protesters wearing face masks was unconstitutional.

With Post wires