Hong Kong’s high court has struck down an emergency ban on face masks that was being used by the police to stop and detain protesters attempting to conceal their identities.

The mask ban, rushed through by Carrie Lam’s government with colonial-era powers that hadn’t been used in decades, was deemed excessive in a ruling handed down on Monday.

Judges said the anti-mask law was unconstitutional, because it infringed on the fundamental rights of citizens more than was reasonably necessary to further its goals.

It represents a victory for protesters, who had largely defied the ban anyway since its imposition in early October, as hundreds battled for control of the Hong Kong Polytechnic University campus.

For days, demonstrators have fortified their position at the university and clashed with lines of riot police officers.

While police are deploying water cannon, teargas and rubber bullets, the protesters have defended the campus with petrol bombs, bricks and bows and arrows. On Sunday, a police media liaison officer was hit in the leg with an arrow fired by protesters.

Police began entering the campus in large numbers on Sunday night, and reports from the scene on Monday suggested many of the protesters now wanted to leave, but were being prevented from doing so.

The situation has been declared a riot by the authorities, meaning those who are arrested face potentially lengthy jail terms. The police, who previously claimed protesters could leave as long as they removed their face coverings, appear to be attempting to break up the protest while taking in as many suspects as they can.

International observers have urged Hong Kong police to treat the situation with restraint, after the authorities threatened to use live ammunition if protesters continued to arm themselves with potentially deadly weapons.

An editor of the Global Times, the English-language arm of the Chinese state-run People’s Daily newspaper, went as far as to suggest police should open fire on the campus with sniper rifles. “If the rioters are killed, the police should not have to bear legal responsibility,” Hu Xijin wrote on Weibo.

In a statement, the university said the clashes had caused the campus to be “severely and extensively vandalised”.

The head of the university, Professor Jin-Guang Teng, tried to persuade the protesters to give up peacefully, saying in a video statement that he had arranged a deal with police whereby he would “personally accompany” them to the police station and “ensure your case is fairly processed”.

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

Ted Hui, an opposition MP who is inside the campus, told reporters there were about 1,000 protesters still inside and that the situation was “dire”, with food supplies running low and an urgent need for medical attention.

He said a bridge that had been posited as a route out for protesters who wanted to leave had been “sealed off” by the police.

Elsewhere in the city, protesters blocked off a nearby major road in an apparent attempt to distract police and help those trapped at the university.

And police issued a “wanted” notice for an injured woman, saying protesters had blocked an ambulance and forcibly freed her from police custody.

Hong Kong has been riven by protests, which escalated in June after the government attempted to pass an unpopular bill that would have made it easier to extradite criminal suspects to mainland China.