HONG KONG (REUTERS) - Hong Kong police threatened on Monday (Nov 18) to use live bullets if "rioters" used lethal weapons and committed other acts of violence, after the latest flare up during five months of anti-government protests in the Chinese ruled city.

The police statement followed fresh clashes outside Hong Kong Polytechnic University (PolyU) where protesters were hunkered down behind makeshift shields and hurled petrol bombs at police in a standoff blocking a vital tunnel link.

Police had said on Sunday one officer had been treated in hospital after being hit in the leg by an arrow and another had his visor struck by a metal ball although he was not hurt.

The violence, which has convulsed the Asian financial hub, has posed the gravest popular challenge to Chinese President Xi Jinping since he came to power in 2012. Xi has said he is confident Hong Kong's government can resolve the crisis.

In Monday's statement, police warned people who were described as rioters to stop using lethal weapons to attack officers and to halt other acts of violence, saying police would respond with force and possibly live bullets if necessary.

Demonstrators, angry at what they see as Chinese meddling in the former British colony, have said they are responding to excessive use of force by police.

"The protesters have been reacting to the police," Joris, 23, a civil engineer, told Reuters. "We haven't fought back as much as we could. I would be prepared for jail. We are fighting for Hong Kong."

Multiple explosions were heard after radical protesters set fire to a key footbridge connecting the campus of Hong Kong Polytechnic University (PolyU) and the Hung Hom MTR station on Sunday evening.

They also threw petrol bombs at police officers on Cheong Wan Road near the MTR station in an attempt to stop the force's dispersal operation. In response, officers fired rounds of tear gas.

On the flyover at the toll booths at the Cross-Harbour Tunnel near PolyU, a police armoured vehicle caught fire after being hit by several petrol bombs thrown by protesters, local broadcaster TVB reported.

The vehicle reversed slowly, according to live-streamed video of the incident shown by several local media outlets, and the blaze was put out by waiting firefighters, said RTHK. Police then fired several rounds of tear gas to disperse the protesters.

Earlier in the day, police said that violent activities at PolyU have escalated to rioting and anyone who stays behind or helps the protesters may be liable to the offence of taking part in a riot.

The escalating violence prompted PolyU to send an urgent notice to its students, staff and alumni, urging everyone to evacuate the campus, public broadcaster RTHK reported. The university also noted that dangerous chemicals were found to be missing from its laboratories.

PolyU said that while it understood students' concerns about the social conditions in Hong Kong, calm and sensibility must prevail when they fight for anything, RTHK reported. The university also condemned the illegal acts of the radical protesters.

Fresh violence erupted around PolyU's campus in Hung Hom on Sunday morning, as protesters continued to attack police officers with improvised weapons such as bricks, petrol bombs as well as bows and arrows. Police responded with rounds of tear gas and deployed an armoured vehicle and two water cannons laced with blue dye.

In the chaos, a police officer was struck in the leg by an arrow shot by a protester around 2pm, while another officer had a small metal ball hit his visor, right above his nose, the city's police force said.

The policeman struck by the arrow was working at the scene on the force's media liaison team. A photograph on police Facebook page showed that the arrow nearly went through his calf. It entered his leg from the back and the tip of the arrow could be seen jutting from under the skin.

The officer was next to the Kowloon Rosary Church at the junction of Chatham Road South and Austin Road, around 30m away from PolyU, when he was hit, police said in a statement.

The injured officer was taken to hospital in a conscious state. A large group of journalists were reporting in the same area when the incident happened. Such an attack poses a grave threat to the safety of everyone at scene, police said.

"Police severely condemn the violent acts of all rioters and are conducting dispersal and arrest operations," the statement said. Police also advised members of the public to avoid going to the vicinity of the university.

The anti-riot police officer whose visor was hit by a small metal ball was not injured, police said in a statement.

Police warned all rioters and their accomplices to stop all illegal acts and leave the campus immediately. The force also appealed to journalists at the scene "to mind their own safety and not to obstruct police action".

Meanwhile, police confirmed that a Long Range Acoustic Device, installed on top of a police armoured vehicle, was deployed on the bridge that heads over the Cross-Harbour Tunnel toll booths, to give warnings to protesters.

The force stressed that it is a broadcasting system that conveys important messages over a long range in a noisy environment. The device is not a weapon, police clarified, and it does not generate ultra-low frequency that causes dizziness, nausea or a loss of sense of direction, as falsely reported in the media.

Police also fired tear gas to try to break up protesters on the artery of Nathan Road in the Kowloon district of Mong Kok. Protesters occupied a number of roads in the area and built barricades with rubbish. They also hurled bricks and petrol bombs at police officers.

Earlier in the day, the city's Education Bureau announced that classes at all schools will continue to be suspended on Monday for the safety of students. All classes at kindergartens, primary and secondary schools as well as special schools have been suspended since last Thursday.

In a statement on Sunday, the bureau said that schools should prepare for classes to resume, but students should stay at home and not take part in any unlawful activities.

"Although the roads and public transport services in the territory have gradually resumed, there are still uncertain factors currently," the bureau said.

The Social Welfare Department also advised members of the public not to take their children to childcare centres and centres providing after-school care programmes on Monday. The advisory also applies to those who go to elderly services centres, day activity centres and day rehabilitation units.

With additional information from Reuters