Hong Kong protesters yesterday apologised for "over-reacting" during heated clashes at the city's airport where suspected Chinese spies were detained and beaten by some demonstrators.

Flights resumed at Hong Kong’s international airport on Wednesday after Tuesday's stand off ended in violent confrontations between pro-democracy protesters and riot police.

Hundreds of flights were cancelled on Tuesday after activists blocked the terminal and clashed with police in chaotic scenes that saw officers fire pepper spray and one draw his gun.

“It is not our intention to cause delays to your travels,” read a formal statement issued by protesters on Wednesday night, “we ask for your understanding and forgiveness as young people in Hong Kong continue to fight for freedom.”

"After months of prolonged resistance, we are frightened, angry and exhausted," it continued. "Some of us have become easily agitated and over-reacted last night."

Beijing seized on the chaotic scenes to condemn the protesters on Wednesday for “near-terrorism” at Hong Kong airport, denouncing them for “violent acts” after they surrounded two men, fuelled by suspicions that they were undercover police or spies.

It later emerged that one was a traveller at the airport, and the other, a reporter for China’s Global Times newspaper, a state media outlet controlled by the Communist Party. China has ramped up its rhetoric of the protests lately, sending ominous signals that military and police officers were ready to deploy at a moment’s notice to restore order.

On Wednesday the US State Department said it was concerned about movements of Chinese forces on the border and urged Beijing to honor the territory's autonomy.

"The United States is deeply concerned by reports of Chinese paramilitary movement along the Hong Kong border," a spokesperson said. "The United States strongly urges Beijing to adhere to its commitments in the Sino-British Joint Declaration to allow Hong Kong to exercise a high degree of autonomy."

China on Wednesday denied requests for two US Navy ships to visit Hong Kong, the Pacific Fleet said on Tuesday.

The USS Green Bay, an amphibious dock landing ship, was to stop in Hong Kong on Saturday, while the guided missile cruiser USS Lake Erie planned a port call there next month. Early this month, Beijing demanded that US diplomats based in Hong Kong "stop interfering" in the city's affairs, after reports that they met with pro-democracy activists.

US President Donald Trump, however, faced criticism at home on Tuesday for avoiding harsh words over Beijing's response to the protesters, who object to sliding freedoms in the semi-autonomous global financial center. A US Navy ship last visited Hong Kong in April.

Foreign Secretary Dominic Raab tweeted Tuesday: “Concerning to see what’s happening in Hong Kong and the worrying pictures of clashes between police & protesters at the airport. As I said to Carrie Lam during my call last week, we condemn the violence & encourage constructive dialogue to find a peaceful way forward.”

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And on Tuesday the United Nations High Commissioner for Human Rights urged Hong Kong to exercise restraint and investigate evidence of its forces firing tear gas at protesters in ways banned under international law.

Security at the airport was tighter than usual on Wednesday and authorities had obtained an injunction preventing any further protests at the site, although there were doubts as to its efficacy.

Some entrances were closed and armed police patrolled by check-in counters in the departures hall.

Airlines were working to rebook thousands of stranded passengers after hundreds of flights were delayed or cancelled this week after demonstrators flooded the departures terminal and arrivals hall.

A handful of demonstrators remained on Wednesday but operations otherwise largely returned to normal. The protesters sat on the ground sharing snacks and waving posters, some of which apologised for the disruption.

The airport has been flooded by activists since last Friday as they argue it is the last safe refuge from tear gas, which cannot be fired inside without hitting foreign tourists.

Last weekend, violence escalated significantly when riot police fired tear gas into a subway station. “It is as a last resort that we have moved the protest to the airport,” demonstrators said in a statement.

“After months of prolonged resistance, we are frightened, angry and exhausted. Some of us have become easily agitated and over-reacted last night.

For this we feel pained and dispirited and would like to express our most sincere apologies.”

British comedian Bill Bailey was caught up in the chaos with his family while on a layover from Bali back to London.

"I think the scale of protests is extraordinary," he told The Telegraph. "We've been coming to Hong Kong for over 20 years now; I've worked here, and we've never seen anything like it."

“Everyone’s been good natured, very polite, very respectful. Protesters came up to us and offered us food and drink, and said 'sorry you’ve had to wait,' and are constantly apologising for the inconvenience, hoping that we understand.”

Hong Kong is facing its worst political crisis since the former British colony was returned to Chinese rule. Millions first came to the streets against an extradition proposal that would have sent people to face trial in mainland China, where the Communist Party controls the courts.

Anger has risen steadily against city leaders for failing to make any concessions and the police for escalating crowd-control tactics, shooting tear gas, rubber bullets and bean bag rounds ending almost all protest in violence.

Last night Hong Kong police fired teargas at hundreds of protesters who had gathered outside a police station in the residential area of Sham Shui Po in Kowloon.

Witnesses at the scene saw police shoot several rounds with little warning at demonstrators who had been shining laser beams at the police station and burning joss papers on the roadside.