Police fire tear gas at anti-extradition bill protesters during clashes in Sham Shui Po in Hong Kong. Photo: Reuters

Hong Kong – China said on Wednesday Hong Kong's protest movement had reached "near terrorism" and more street clashes followed ugly scenes the previous day when protesters set upon men they suspected of being government sympathisers.

The United States said it was deeply concerned at news of Chinese police forces gathering near the border, urged Hong Kong's government to respect freedom of speech and issued a travel advisory urging caution when visiting the city.

By nightfall, police and protesters were again facing off on the streets, with riot officers shooting tear gas almost immediately as their response to demonstrators toughens.

Ten weeks of increasingly violent confrontation between police and protesters have plunged Hong Kong into its worst crisis since it reverted from British to Chinese rule in 1997.

Flights resumed on Wednesday amid heightened security at Hong Kong airport, which is one of the world's busiest, after two days of disruptions sparked by protesters swarming the airport and, late Tuesday, detaining there two men they suspected opposed them.

China's Hong Kong and Macau Affairs Office in Beijing called the behaviour at the airport no different to terrorism and said it must be severely punished.

"We're deeply sorry about what happened yesterday," read a banner held up by a group of a few dozen demonstrators in the airport arrivals hall in the morning.

"We were desperate and we made imperfect decisions. Please accept our apologies," the banner said.

In chaotic scenes that would once have been unthinkable for Hong Kong, a peaceful sit-in at the airport turned violent late on Tuesday as protesters confronted and held a man they believed was an undercover Chinese agent.

Busloads of riot police arrived in response, clashing with furious demonstrators before withdrawing once the man was removed, and leaving the terminal briefly in control of activists who then detained a Chinese reporter for a short time.

It was not clear whether the scenes of violence might have eroded the broad support the movement has so far attracted in Hong Kong, a major financial hub. The protests have also hit the city's faltering economy.

"We promise to reflect and to improve," protesters said in one message distributed on social media app Telegram.

"Sorry we were too reckless… we are only afraid of losing your support to the whole movement due to our mistake, and that you give up on fighting."

They also showed little sign of relenting in their protests, which began in opposition to a now-suspended bill that would have allowed the extradition of suspects for trial in mainland China, but have swelled into wider calls for democracy.

Hundreds attended a demonstration in the residential area of Sham Shui Po, where police arrived and quickly used tear gas after protesters pointed lasers at the police station.

China used its strongest language yet after Tuesday's incidents, when the protesters seized a reporter from China's Global Times newspaper, a nationalistic tabloid run by the ruling Communist Party's official People's Daily, and harassed the man they believed to be a mainland agent.

In addition to Beijing's condemnation, the People's Daily called for "using the sword of the law" to restore order and mainland social media users lauded the detained reporter as a hero.

The US State Department called for restraint and said the country was deeply concerned at Chinese paramilitary movements near the border.

"The ongoing demonstrations in Hong Kong reflect the sentiment of Hong Kongers and their broad and legitimate concerns about the erosion of Hong Kong's autonomy," a department spokeswoman said.

Under the "one country-two systems" arrangement, Hong Kong was guaranteed a high degree of autonomy and human rights after its handover to China.

Any erosion of the city's autonomy could undermine its preferential trade treatment by the United States, the spokeswoman said.

Chinese police have assembled in the neighbouring city of Shenzhen for what appeared to be exercises, the Global Times reported this week.

On Tuesday, US President Donald Trump had described the volatile situation as "tricky" and said China's government had moved troops near the border with Hong Kong.

China has also denied a request for two US Navy warships to visit Hong Kong in the coming weeks, US officials said.

Beijing and Washington are already mired in a trade dispute.

France urged Hong Kong authorities to renew dialogue with protesters to find a peaceful solution.

At Hong Kong airport, which was designed by renowned British architect Norman Foster, blood, debris and signs of the scuffle were scrubbed away during the night, and cleaners and protesters themselves removed anti-government posters from the walls.

Hong Kong carrier Cathay Pacific Airways said a total of 272 departures and arrivals had been cancelled because of the disturbances, affecting more than 55 000 passengers.

China's aviation regulator demanded last week that Cathay suspend personnel supporting protests in Hong Kong from staffing flights entering its airspace. On Wednesday, the carrier said it had fired two pilots.

Forward Keys, a flight data firm, said the crisis had driven a 4.7 percent fall in long-haul bookings to Hong Kong between June 16 and August 9 compared with the same period last year.

"I think the local events clearly are having a profound impact, probably in ways that we haven’t necessarily clearly articulated yet," Charles Li, chief executive of the Hong Kong Stock Exchange told reporters on Wednesday.

The protesters vowed to press on. "All the people here are very scared," Ann, a 21-year-old teacher, told Reuters at the airport as she carefully took down anti-government posters, folding them for re-use.

"But we are more scared that we do not have our freedoms any more, and so that is why we continue our protests," she said.

Reuters