Hong Kong ​police have stormed into a university campus held by protesters after an all-night stand-off.

Fiery explosions could be seen inside as riot officers entered before dawn on Monday.

Police had fired repeated barrages of tear gas and water cannon at protesters outside the campus since before midnight.

Anti-government protesters had barricaded themselves inside Hong Kong Polytechnic University for days, fighting back with petrol bombs and bows and arrows.

Police threatened early on Monday morning to use live ammunition on protestors after an officer was skewered by an arrow on Sunday - marking a severe escalation after more than half a year of protest.

Hong Kong protesters out in force after months of turmoil: In pictures Show all 32 1 /32 Hong Kong protesters out in force after months of turmoil: In pictures Hong Kong protesters out in force after months of turmoil: In pictures A protester throws a stone into a building at Hong Kong Polytechnic University on 11 November Reuters Hong Kong protesters out in force after months of turmoil: In pictures Police patrol the streets of Hong Kong the day after an officer shot a protester during a protest on 11 November EPA Hong Kong protesters out in force after months of turmoil: In pictures Riot police stand guard during a protest against police brutality in Hong Kong on 27 October EPA Hong Kong protesters out in force after months of turmoil: In pictures Protesters carry traffic cones to build a barricade during anti-government protests in Causeway Bay, Hong Kong on 11 November Reuters Hong Kong protesters out in force after months of turmoil: In pictures Protesters use obstacles and bricks to block a road in Hong Kong on 11 November AP Hong Kong protesters out in force after months of turmoil: In pictures Graffiti on a window smashed by student protesters in Hong Kong Polytechnic University AP Hong Kong protesters out in force after months of turmoil: In pictures Protesters carry US and British flags during a demonstration in Hong Kong on 11 November Reuters Hong Kong protesters out in force after months of turmoil: In pictures A protester throws a bottle of water as he is shrouded in tear gas during a demonstration in the Tsim Sha Tsui district of Hong Kong on 27 October Reuters Hong Kong protesters out in force after months of turmoil: In pictures Journalists take pictures of police officers as they move to disperse protesters in Hong Kong on 27 October Reuters Hong Kong protesters out in force after months of turmoil: In pictures An man lies injured after being attacked by protesters who suspected him of being an undercover police officer on 27 October EPA Hong Kong protesters out in force after months of turmoil: In pictures A man runs among tear gas during a protest in Hong Kong on 27 October Reuters Hong Kong protesters out in force after months of turmoil: In pictures Protesters gather on a field in the Tsim Sha Tsui district of Hong Kong on 27 October Reuters Hong Kong protesters out in force after months of turmoil: In pictures Pro-democracy protesters turn away from a fire lit outside the Causeway Bay Mass Rapid Transit (MTR) station in Hong Kong on 4 October Getty Hong Kong protesters out in force after months of turmoil: In pictures People rest near rows of riot police officers during a protest in Mong Kok, Hong Kong on 27 October Reuters Hong Kong protesters out in force after months of turmoil: In pictures Pro-democracy protesters hold umbrellas as police fire tear gas at them ouside Tai Koo MTR station in Hong Kong on 3 October Getty Hong Kong protesters out in force after months of turmoil: In pictures Hong Kong police fire a water cannon from the central government office at protesters during a mass rally on the 70th anniversary of the founding of communist China on October 1 AFP/Getty Hong Kong protesters out in force after months of turmoil: In pictures Protesters burn a Chinese national flag during a mass rally on the 70th anniversary of the founding of communist China on 1 October AFP/Getty Hong Kong protesters out in force after months of turmoil: In pictures Protesters remove signs celebrating the 70th anniversary of the founding of communist China during a mass rally in Hong Kong on 1 October AFP/Getty Hong Kong protesters out in force after months of turmoil: In pictures Cardboard boxes set alight by protesters burn in the streets of Hong Kong during a mass rally on the 70th anniversary of the founding of communist China on 1 October EPA Hong Kong protesters out in force after months of turmoil: In pictures An anti-China banner has been placed in a barricade during a mass rally on the 70th anniversary of the founding of communist China on 1 October EPA Hong Kong protesters out in force after months of turmoil: In pictures A pro-democracy protester runs away after police fire a tear gas canister in Hong Kong's Wong Tai Sin district on October 4 Reuters Hong Kong protesters out in force after months of turmoil: In pictures Protesters run after police fire tear gas during a mass rally on the 70th anniversary of the founding of communist China on 4 October AFP/Getty Hong Kong protesters out in force after months of turmoil: In pictures Pro-democracy protester take cover after police fire a tear gas canister in Hong Kong's Wong Tai Sin district on 4 October Reuters Hong Kong protesters out in force after months of turmoil: In pictures The rally comes after months of protests on the streets of Hong Kong which began in oppposition to a proposed extradition bill EPA Hong Kong protesters out in force after months of turmoil: In pictures The extradition bill would have allowed the government to extradite people to China if they were facing certain criminal charges AFP/Getty Hong Kong protesters out in force after months of turmoil: In pictures Opposition to the bill stems from the fear that the Chinese government would abuse this power for political or commercial reasons EPA Hong Kong protesters out in force after months of turmoil: In pictures Protesters soon came to demand greater freedom and universal suffrage under the One Country, Two Systems principle AFP/Getty Hong Kong protesters out in force after months of turmoil: In pictures Protesters are wearing black to symbolise opposition to China as they take to the streets in a "day of grief" while the Chinese state celebrates the 70th anniversary of its communist founding AFP/Getty Hong Kong protesters out in force after months of turmoil: In pictures A protester charges forward holding umbrellas as a mass rally breaks out in violence in Hong Kong on 1 October AFP/Getty Hong Kong protesters out in force after months of turmoil: In pictures A protester pours water on a tear gas canister fired by police during a mass rally on the 70th anniversary of the founding of communist China on 1 October Reuters Hong Kong protesters out in force after months of turmoil: In pictures A protester carries a vandalised Chinese flag through Hong Kong during a mass rally on the 70th anniversary of the founding of communist China on 1 October EPA Hong Kong protesters out in force after months of turmoil: In pictures Protesters carry a banner that denounces the celebration of the 70th anniversary of the founding of communist China on 1 October EPA

The continuing conflict around the semi-autonomous state's universities has seen protesters set a bridge on fire to keep police - armed with tear gas and water cannon - from advancing on their campus stronghold.

Police said the arrow struck a media liaison officer in the calf on Sunday and he was taken to a hospital. Photos on the department’s Facebook page show the arrow sticking out of the back of the officer’s leg.

Now officers have said they will consider the use of live bullets - ammunition which has only been deployed fleetingly throughout the protest, including one incident when an off-duty officer fired on a crowd, striking a boy in the leg.

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

As riot police moved in on the university from all sides, some protesters retreated inside Hong Kong Polytechnic University while others set fires on bridges leading to it.

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 port that has been blocked by the protesters for days.

Protesters are trying to keep the pressure on Hong Kong leaders, who have rejected most of their demands.

The protests were sparked by proposed legislation that would have allowed the extradition of criminal suspects to the mainland.

Activists saw it as an erosion of Hong Kong’s autonomy under the “one country, two systems” formula implemented in 1997, when the UK returned the territory to China.

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.

Police and protesters faced off all day on Sunday after a pitched battle the previous night in which the two sides exchanged tear gas and petrol bombs that left fires blazing in the street.

A large group of people arrived in the morning to try to clean up the road but were warned away by protesters.

Riot police shot several volleys of tear gas at the protesters, who sheltered behind a wall of umbrellas and threw petrol bombs into nearby bushes and trees, setting them on fire.

The protesters held their ground for most of the day, as water cannon trucks drove over bricks and nails strewn by protesters to spray them at close range – some with water dyed blue to help police identify protesters afterwards.

The officer was struck in the calf as police faced off against protesters (EPA)

Protesters began retreating into the university near sunset, fearing they would be trapped as police fired tear gas volleys and approached from other directions.

The protesters have barricaded the entrances to the campus and set up narrow access control points.

They are the holdouts from larger groups that occupied several major campuses for much of last week.

Another group threw bricks in the street to block a main thoroughfare in the Mongkok district, as police fired tear gas to try to disperse them.

The disruption to traffic on Nathan Road may have been an attempt to distract police during the standoff at the polytechnic.

Opposition politicians criticised the Chinese military for joining a cleanup to remove debris from streets near Hong Kong Baptist University on Saturday.

Dozens of Chinese troops, dressed in black shorts and olive drab T-shirts, ran out in loose formation and picked up paving stones, rocks and other obstacles that had cluttered the street.

The military is allowed to help maintain public order but only at the request of the Hong Kong government.

The government said it had not requested the military’s assistance, describing it as a voluntary community activity.