Hong Kong protesters have shut down one of the world’s busiest airports in a dramatic escalation of the mass demonstrations that have plunged the city into its worst political crisis in decades.

The unprecedented cancellation of all flights followed the fourth consecutive day of protests at the airport and amid increasingly threatening statements from Beijing. A Chinese official said “terrorism” was emerging in the city, while in Hong Kong authorities demonstrated water cannon for use in crowd control.

The protests are in their 10th week, with confrontations between protesters and police growing more violent. Rights groups and democracy activists have accused police of using increasingly excessive force. At least 40 people were treated in hospital after clashes on Sunday, including a woman who was hit, reportedly with a beanbag round, and could potentially lose an eye.

Facebook Twitter Pinterest An injured young woman receives medical assistance after being hit in one eye during a demonstration in Hong Kong. Photograph: Issei Kato/Reuters

Protesters in black T-shirts and face masks filled the airport, handing out lists to arriving visitors documenting alleged police violence and holding up graphic images of injured protesters. Some held signs that said: “An eye for an eye” and wore eye patches in solidarity with the injured woman.

Others held posters that said: “Hong Kong is not safe” and “Shame on police” and chanted: “Stand with Hong Kong, fight for freedom!”

“I just don’t understand how people can tolerate that kind of police brutality. I feel like if I don’t come out now, I can’t come out ever,” said Hilary Lo, who took a half day’s sick leave from her accountancy firm to attend the demonstration.

“People are starting to realise the police are out of control, especially with what has happened in the past two weeks,” she said.

Play Video 1:19 Police fire teargas into Hong Kong subway station – video

Tourists remained at the airport through the protest, with flights expected to resume at 6am on Tuesday. Elodichukwu Obiageli, from Nigeria, said she had been stranded for five hours. “We had no information from our airline. We are just stranded here – we have no money,” she said, adding that all airport stores had closed.

By the early evening, crowds had thinned amid reports police would move in to clear the airport but when they did not show, thousands of protesters streamed back, bringing supplies to stay through the night.

“Honestly, I don’t think anything will happen,” said Andy Chu, a protester who remained at the airport. “I think the police strategy until now we can see is to burn out our energy, just let us sit here and wait.”

“A few hours ago there were rumours flying around, saying the police are coming in to kick us out, with teargas,” he said. “I think that is also from the police. That’s part of their tactics, part of the game. They want most of the more peaceful protesters to leave themselves.”

Quick guide What are the Hong Kong protests about? Show Hide Why are people protesting? The protests were triggered by a controversial bill that would have allowed extraditions to mainland China, where the Communist party controls the courts, but have since evolved into a broader pro-democracy movement. Public anger – fuelled by the aggressive tactics used by the police against demonstrators – has collided with years of frustration over worsening inequality and the cost of living in one of the world's most expensive, densely populated cities. The protest movement was given fresh impetus on 21 July when gangs of men attacked protesters and commuters at a mass transit station – while authorities seemingly did little to intervene. Underlying the movement is a push for full democracy in the city, whose leader is chosen by a committee dominated by a pro-Beijing establishment rather than by direct elections.

Protesters have vowed to keep their movement going until their core demands are met, such as the resignation of the city’s leader, Carrie Lam, an independent inquiry into police tactics, an amnesty for those arrested and a permanent withdrawal of the bill. Lam announced on 4 September that she was withdrawing the bill. Why were people so angry about the extradition bill? Beijing’s influence over Hong Kong has grown in recent years, as activists have been jailed and pro-democracy lawmakers disqualified from running or holding office. Independent booksellers have disappeared from the city, before reappearing in mainland China facing charges. Under the terms of the agreement by which the former British colony was returned to Chinese control in 1997, the semi-autonomous region was meant to maintain a “high degree of autonomy” through an independent judiciary, a free press and an open market economy, a framework known as “one country, two systems”. The extradition bill was seen as an attempt to undermine this and to give Beijing the ability to try pro-democracy activists under the judicial system of the mainland. How have the authorities responded? Beijing has issued increasingly shrill condemnations but has left it to the city's semi-autonomous government to deal with the situation. Meanwhile police have violently clashed directly with protesters, repeatedly firing teargas and rubber bullets. Beijing has ramped up its accusations that foreign countries are “fanning the fire” of unrest in the city. China’s top diplomat Yang Jiechi has ordered the US to “immediately stop interfering in Hong Kong affairs in any form”. Lily Kuo and Verna Yu in Hong Kong

Hong Kong’s summer of dissent has presented one of the biggest challenges to China’s leader, Xi Jinping, since he came to power in 2012. Yang Guang, a spokesman for the Hong Kong and Macao Affairs Office of the State Council called on authorities to “show no mercy” in dealing with the protesters.

“Hong Kong’s radical demonstrators have repeatedly used extremely dangerous tools to attack police officers, which already constitutes a serious violent crime, and also shows the first signs of terrorism emerging,” Yang said at a press briefing. “This wantonly tramples on Hong Kong’s rule of law and social order.”

State-backed media in China on Monday said armed police had held exercises in the neighbouring city of Shenzhen.

In an apparent warning to protesters of a toughening approach on the part of authorities, Hong Kong police invited legislators and journalists on Monday to witness a display of water cannon. Police have never used the device since two were bought after pro-democracy protests in 2014, but during Monday’s demonstration one was blasted at dummy targets in a training facility.

Man-Kei Tam, the director of Amnesty International’s Hong Kong division, warned that clashes between protesters and police had “escalated to another level, especially on the police side” over the weekend.

Facebook Twitter Pinterest An anti-riot vehicle equipped with water cannon sprays water on a dummy during a demonstration in Hong Kong on Monday. Photograph: Kin Cheung/AP

Tam cited footage of police firing teargas into a subway station in Kwai Fong on Sunday night. It was not clear how many protesters were in the station but it was rare for officers to fire teargas indoors. He also shared a video of police firing non-lethal projectiles at close range as protesters attempted to flee down an escalator at another subway station.

The police have also reported injuries among their ranks, including eye irritation from laser pointers and petrol bomb burns.

Civil Rights Observer, a local rights group that sends observers to protests, said it had serious concerns about police violence and had seen “clear evidence to show the police are violating their guidelines”, according to its spokesman, Icarus Wong Ho-yin.

During the protests at the weekend, the Hong Kong Free Press news website posted footage of one arrest that appeared to show officers dressed as protesters pressing a demonstrator to the ground. The young man, who said his name was Chow Ka-lok and asked for a lawyer, sustained head wounds and a broken tooth.

Protests in Hong Kong began in early June against a legislative bill that would have allowed for residents to stand trial in mainland China on criminal charges. While the territory returned to Chinese rule in 1997, it was promised semi-autonomy for 50 years including a separate legal system. Many protesters feared the bill, now suspended, would have led to the decline of civil and political rights in the Asian financial hub.