Large numbers of Chinese paramilitary forces have been filmed assembling just 30km (18.6 miles) from Hong Kong in the city of Shenzhen, as riot police clashed with protesters occupying the airport for a second day.

The scuffles broke out in the evening between police and protesters, after paramedics attempted to reach an injured man whom protesters had detained on suspicion of being an undercover agent.

The burst of violence included protesters beating up at least two men they suspected of being undercover agents. Police have acknowledged using “decoy” officers, and the violence followed weekend sightings of men dressed like demonstrators – in black and wearing face masks – appearing to arrest protesters.

In both instances, angry demonstrators pushed past people trying to hold them back and attacked the men, binding their wrists together and beating them to the ground. The two were eventually taken away by paramedics.

In one case, protesters detained a man they claimed to be an undercover police officer from mainland China, pulled his identity documents from his wallet and encouraged journalists to photograph them. None of them showed that he was a police officer, although protesters claimed to have found his name on an online list of police officers in southern Guangdong province.

Sally Tong, an 18-year-old protester, said they needed to hold him as evidence that mainland Chinese authorities are in Hong Kong to monitor the demonstrations. Tong said the man was dressed in black and wore a mask to look like one of them. “We want to keep him here and investigate,” Ms Tong said.

The protesters also apprehended another man from mainland China. But they could not agree on who they believed he was: some said he was a gangster, others said he was a fake reporter, and still others said he was masquerading as a protester. As with the first man, some protesters tied his wrists together and poured cold water over his head. Airport security appeared unable to control the crowd.

Hu Xijin, editor-in-chief of the Global Times, a nationalistic Chinese tabloid, said the man was one of his reporters. “Fu Guohao, reporter of GT website is being seized by demonstrators at HK airport,” Hu wrote on his widely-followed Twitter account. “I affirm this man being tied in this video is the reporter himself. He has no other task except for reporting.”

Several police vehicles were also blocked by protesters, and riot police moved in, pushing some protesters back and using pepper spray and batons at times. Demonstrators reacted by throwing bottles and other projectiles. A number of people were arrested.

In other footage of the clash shared around social media, protesters surrounded an armed officer who had forced a woman to the ground, grabbed his baton and beat him with it. The officer then drew his pistol and the protesters ran off.

After a brief period early in the day when flights were able to take off and land, the airport authority suspended check-in services for departing flights as of 4.30pm Hong Kong time. Departing flights that had completed the process would continue to operate.

Continuing protests rock Hong Kong: In pictures Show all 35 1 /35 Continuing protests rock Hong Kong: In pictures Continuing protests rock Hong Kong: In pictures Police officers point their guns towards pro-democracy protesters after a clash at a march in Hong Kong on August 25 Reuters Continuing protests rock Hong Kong: In pictures Pro-democracy protesters throw molotov cocktails during clashes with riot police in Hong Kong on August 25 Reuters Continuing protests rock Hong Kong: In pictures Demonstrators rally behind barricades ass they are shrouded in tear gas deployed by police in Hong Kong on August 25 Reuters Continuing protests rock Hong Kong: In pictures Police clash withpro-democracy protesters at a march in Hong Kong on August 25 Reuters Continuing protests rock Hong Kong: In pictures A police officer prepares to strike a protester as clashes erupt during a pro-democracy march in Hong Kong on August 24 AFP/Getty Continuing protests rock Hong Kong: In pictures A water cannon is fired by riot police to clear the street of demonstrators during a protest in Hong Kong on August 25 EPA Continuing protests rock Hong Kong: In pictures Pro-democracy protesters use a slingshot during clashes with riot police in Hong Kong on August 25 Reuters Continuing protests rock Hong Kong: In pictures Riot police fire tear gas to disperse pro-democracy protesters in Hong Kong on August 24 Reuters Continuing protests rock Hong Kong: In pictures A demonstrator throws a molotov cocktail as they clash with riot police during a protest in Hong Kong on August 25 Reuters Continuing protests rock Hong Kong: In pictures A demonstrator swings a tennis racket in an attempt to knock back a tear gas cannister thrown by riot police during a protest in Hong Kong on August 25 Reuters Continuing protests rock Hong Kong: In pictures A protester displays a Hong Kong Independence flag during a pro-democracy march in Hong Kong on August 24 EPA Continuing protests rock Hong Kong: In pictures Riot police tend to a detained demonstrator who has been injured during a protest in Hong Kong on August 25 Reuters Continuing protests rock Hong Kong: In pictures A demonstrator uses bamboo poles to build a barricade during a protest in Hong Kong on August 25 Reuters Continuing protests rock Hong Kong: In pictures Riot police clash with demonstrators during a protest in Hong Kong on August 24 Reuters Continuing protests rock Hong Kong: In pictures Protesters are hit by tear gas fired by the police in Hong Kong on August 24 AFP/Getty Continuing protests rock Hong Kong: In pictures Demonstrators shrouded in tear gas shield themselves behind barricades during a clash with riot police in Hong Kong on August 25 Getty Continuing protests rock Hong Kong: In pictures Riot police arrest a protester during a pro-democracy march in Hong Kong on August 24 EPA Continuing protests rock Hong Kong: In pictures Police clash with pro-democracy protesters at the Hong Kong international airport on August 13 AFP/Getty Continuing protests rock Hong Kong: In pictures Protesters fall back as they clash with police during a protest at Kowloon Bay in Hong Kong on August 24 AFP/Getty Continuing protests rock Hong Kong: In pictures Police clash with pro-democracy protesters at the Hong Kong international airport on August 13 Reuters Continuing protests rock Hong Kong: In pictures Riot police move to disperse protesters during a clash at the Hong Kong international airport on August 13 Reuters Continuing protests rock Hong Kong: In pictures Commuters riding in a bus drive past protesters in Hong Kong's Kowloon Bay on August 24 AFP/Getty Continuing protests rock Hong Kong: In pictures Protester hold umbrellas as they take part in a pro-democracy march in Hong Kong on August 24 EPA Continuing protests rock Hong Kong: In pictures Riot police detain a demonstrator as they clash during a protest in Hong Kong on August 24 Reuters Continuing protests rock Hong Kong: In pictures Pro-democracy protesters block the entrance to the airport terminals after clashes with police at the Hong Kong international airport on August 13 AFP/Getty Continuing protests rock Hong Kong: In pictures Riot police spray pepper spray as shield themselves by a police van during a clash with pro-democracy protesters at the Hong Kong international airport on August 13 Reuters Continuing protests rock Hong Kong: In pictures Riot police use pepper spray to disperse protesters during a demonstration at the Hong Kong international airport on August 13 Reuters Continuing protests rock Hong Kong: In pictures Demonstrators shrouded in tear gas shield themselves behind barricades during a clash with riot police in Hong Kong on August 25 Reuters Continuing protests rock Hong Kong: In pictures Medical professionals gather to protest police brutality at the Queen Elizabeth Hospital in Hong Kong on August 13 Getty Images Continuing protests rock Hong Kong: In pictures Police detain a pro-democracy protester at the Hong Kong international airport on August 13 AFP/Getty Continuing protests rock Hong Kong: In pictures A protester is halted by police during an occupation at the Hong Kong international airport on August 13 AP Continuing protests rock Hong Kong: In pictures Police clash with pro-democracy protesters at the Hong Kong international airport on August 13 AFP/Getty Continuing protests rock Hong Kong: In pictures Police stand guard at the entrance of the departure terminals at the Hong Kong international airport on August 13 AFP/Getty Continuing protests rock Hong Kong: In pictures Protesters continue their occupation at the Hong Kong international airport on August 14 AP Continuing protests rock Hong Kong: In pictures Travellers read placards and posters left by pro-democracy protesters at the Hong Kong international airport on August 14 AP

The airport authority said it did not expect arriving flights to be affected, although dozens were already cancelled. The authority advised people not to come to the airport, one of the world’s busiest transport hubs.

Hong Kong’s pro-Beijing leader Carrie Lam said on Tuesday the city had been placed on a “path of no return” after 10 weeks of increasingly disruptive protests, as the UN warned Beijing to exercise restraint in its response to growing unrest in the territory.

Chinese state media described the build-up of armed police units, shown in videos gathering at an arena called the Shenzhen Bay Sports Centre, as preparations for “apparent large-scale exercises”. Alexandre Krauss, a policy adviser for the EU’s Committee on Foreign Affairs, called the videos a sign that “something extraordinarily bad is about to happen”.

Similar exercises on 6 August featured up to 12,000 troops, according to the Global Times newspaper, and featured armoured personnel carriers, helicopters and amphibious vehicles.

The newspaper described the People’s Armed Police forces as being mandated by Chinese law for “dealing with rebellions, riots, serious violent and illegal incidents, terrorist attacks and other social security incidents”.

The show of force is a further sign of Beijing’s waning patience with the unrest in Hong Kong, after the Chinese government said on Monday that the protest movement in the city had begun to reveal “sprouts of terrorism”.

While China defines terrorism loosely, it has previously used the term to describe non-violent opposition movements in minority regions such as Tibet and Xinjiang, justifying greater uses of force and the suspension of legal rights for detainees.

Speaking on BBC radio, Britain’s last governor of the city before the 1997 handover said it would be “a catastrophe for China and of course for Hong Kong” if there was a military intervention.

Chris Patten said it was counterproductive of China to warn of “other methods” if the protests did not stop. “Since President Xi has been in office, there’s been a crackdown on dissent and dissidents everywhere, the party has been in control of everything,” he said.

“I very much hope that even after 10 weeks of this going on, the government and President Xi [Jinping] will see the sense in establishing a way of actually bringing people together,” Lord Patten said.

In Washington, Donald Trump said that he hoped the “tricky” situation in Hong Kong would work out “for liberty”.

“It’s a very tricky situation. I think it will work out and I hope it works out, for liberty. I hope it works out for everybody, including China. I hope it works out peacefully. I hope nobody gets hurt. I hope nobody gets killed,” Mr Trump said.

He later tweeted that US intelligence has told him that the Chinese government is moving troops near to Hong Kong and that: “Everyone should be calm and safe!”

Steve Tsang, director of the China Institute at Soas University of London, said that despite repeated shows of force “we are still some distance from [Chinese] security forces being deployed in Hong Kong”.

“But it is much closer today than a month ago,” he added, when protesters targeted the main central government headquarters in Hong Kong and a Chinese flag was defaced.

Mr Tsang said the shift in China’s perception of the protests, rather than its troop movements, was the critical issue. “Beijing now sees events in Hong Kong as a ‘colour revolution’… part of an American-led global conspiracy which aims ultimately at regime change in China,” he said. “This is totally intolerable to Xi Jinping.”

In a statement, the UN high commissioner for human rights, Michelle Bachelet, said that by conflating the Hong Kong protests with “terrorism”, China risked inflaming the situation. She urged the authorities to exercise restraint and to investigate evidence of uses of excessive force by police – one of the protesters’ key demands.

“Officials can be seen firing teargas canisters into crowded, enclosed areas and directly at individual protesters on multiple occasions, creating a considerable risk of death or serious injury,” Ms Bachelet said, referring to videos of recent clashes in the city.

Responding to the UN’s call for restraint, China rejected what it called a “wrongful statement” and said that it sent the “wrong signal to violent criminal offenders”. China’s mission to the UN in Geneva claimed in a statement that Hong Kong protesters had smashed public facilities, paralysed the airport, blocked public transport and used lethal weapons, “showing a tendency of resorting to terrorism”.

“The Chinese central government firmly supports Hong Kong chief executive Carrie Lam and the HK SAR government in discharging their duties in compliance with the law, and supports the Hong Kong police force and judicial organs in enforcing the law decisively,” it said.

Ms Lam also reiterated her support for the police and their tactics, saying they have had to make on-the-spot decisions under difficult circumstances, using “the lowest level of force”.

During angry exchanges in which reporters repeatedly shouted over her, Ms Lam said dialogue would only resume “after the violence has been stopped, and the chaotic situation that we are seeing… subside[s]”.