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.
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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, tear gas 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.
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.
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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”.
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.
The bill has since been withdrawn, but the movement has expanded its demands to include greater democratic freedoms and an amnesty for arrested protesters.