Hong Kong police threatened on Monday to respond with live bullets if "rioters" used lethal weapons and committed other acts of violence, amid a standoff with protesters who have been hurling petrol bombs outside a university campus.

Key points: The officer shot in the leg was from the media department

The officer shot in the leg was from the media department Protests have seen petrol bombs thrown as those demonstrating dig in around the Polytechnic University

Protests have seen petrol bombs thrown as those demonstrating dig in around the Polytechnic University Police fired back with tear gas and water cannons

Police issued a statement urging people they described as rioters to stop using lethal weapons to attack officers at Hong Kong Polytechnic University, adding that police would respond with force and could use bullets if they did not, Reuters reported.

It follows a Hong Kong police officer being admitted to hospital after being shot in the leg by an arrow on Sunday afternoon.

GRAPHIC CONTENT WARNING: Some readers may find images in this story confronting.

Photos on the department's Facebook page showed the arrow sticking out of the back of the officer's lower leg through his pants, though the arrow did not come out the other side of his leg.

The arrow was then snapped off in half. ( Facebook: Hong Kong Police Force )

Authorities used tear gas and water cannons to try and drive back protesters occupying the university and surrounding streets before protesters set bridges leading to the university alight to prevent advancing riot police from reaching them.

Huge fires had lit up the sky at the university in the heart of Kowloon district as protesters hurled petrol bombs, some by catapult, and police fired volleys of tear gas to draw them onto the open podium of the red-brick campus.

Parts of the campus looked more like a fortress with barricades and black-clad protesters manning the ramparts with improvised weapons-like bricks, crates of fire bombs, and bows and arrows at the ready.

A huge blaze burned along much of a long footbridge that connects a train station to the campus over the approach to the Cross-Harbour Tunnel, a major road under Hong Kong's harbour that has been blocked by the protesters for days.

Police said the protesters had now become rioters.

The arrow did not go all the way through the leg. ( Facebook: Hong Kong Police Force )

"Rioters continue to launch hard objects and petrol bombs with large catapults at police officers," police said in a statement.

"The shooting range of such large catapults can reach up to 40 metres … Police warn that the violent activities in the Hong Kong Polytechnic University have escalated to rioting."

'If we don't fight, Hong Kong will be over'

A protester prepares to let an arrow fly. ( Reuters: Athit Perawongnetha )

On the road leading to the university, police vehicles with water cannons advanced on barricades set up by protesters but pulled back when petrol bombs were thrown.

The stand-off blocked the tunnel linking Kowloon to Hong Kong island.

Loading

Officers launched an operation to flush out protesters from inside the university, firing tear gas late on Sunday after the police ultimatum for them to come out expired.

Police created a cordon around the campus to prevent protesters from escaping as they moved in.

"We've been trapped here, that's why we need to fight until the end," Ah Lung, a 19-year-old protester, said.

"If we don't fight, Hong Kong will be over."

Many protesters wore gas masks or tied handkerchiefs over their mouths and noses to protect them from clouds of tear gas.

Some stripped down to their underwear, after earlier dousings from water cannons, one of which used blue-dyed water to drench the protesters.

Dyed water was used on the protesters. ( Reuters: Thomas Peter )

It has also been alleged that police arrested dozens of medical aid volunteers amid the latest unrest.

Loading

Photos posted on social media show lines of volunteers in hi-vis vests detained on the pavement with their hands clasped together and secured with cable ties.

An armoured police vehicle that was set ablaze by petrol bombs in Sunday's violence was towed away early on Monday.

Several blocks from the university, black-clad protesters gathered in Nathan Road, another major thoroughfare, digging up pavements and using bricks to block roads.

The demonstrators shouted: "Liberate HK, revolution of our time."

Police had said on Sunday that police had fired a bullet but did not give details about the latest use of live ammunition.

A protester was last shot on November 11, when an officer fired at an 18-year-old man at point-blank range.

Chinese soldiers in a base close to the university were seen on Sunday monitoring developments with binoculars, Reuters witnesses reported.

Hong Kong continues to test Beijing's patience

Streets in Hong Kong were set alight by Molotov cocktails. ( Reuters Athit Perawongmetha )

The Asian financial hub has been rocked by months of demonstrations, with many people angry at perceived Communist Party meddling in the former British colony, which was guaranteed its freedoms when it returned to Chinese rule in 1997.

Clashes between protesters and police have become increasingly violent in the Chinese-ruled city, which is grappling with its biggest political crisis in decades.

The Education Bureau announced that classes from kindergarten to high school would be suspended again on Monday because of safety concerns.

Classes have been cancelled since Thursday, after the bureau came under criticism for not doing so earlier and the University of Sydney on Friday sent communications to Australian exchange students in Hong Kong to return home.

The demonstrations pose the gravest popular challenge to Chinese President Xi Jinping since he came to power in 2012.

President Xi has said he is confident the Hong Kong Government can resolve the crisis. Until Saturday, Chinese troops in the city had remained inside their base during the protests.

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 currently Hong Kong's most urgent task".

AP/Reuters