China's Hong Kong Liaison office says anti-government protesters are no different to "terrorists", as US President Donald Trump said Chinese troops were moving to the border with Hong Kong and urged calm.
The Beijing-based Hong Kong and Macau Affairs office on Wednesday said extremely violent crimes must be severely punished in accordance with the law.
The strongly worded statements by China's central government follow violent clashes between black-clad protesters and riot police at Hong Kong's international airport, which saw hundreds of flights halted for a second day in one of the world's busiest transit hubs.
A few dozen protesters remained at the airport on Wednesday while workers scrubbed it clean of blood and debris. Check-in counters reopened to queues of hundreds of weary travellers who had waited overnight for their flights.
Ten weeks of increasingly violent clashes between police and pro-democracy protesters, angered by a perceived erosion of freedoms, have plunged the Asian financial hub into its worst crisis since it reverted from British to Chinese rule in 1997.
China's Hong Kong and Macau Affairs Office said it strongly condemned the "near terrorism criminal actions" in Hong Kong including what it called a violent attack on a mainland Chinese journalist and tourist at the airport.
Police condemned violent acts by protesters overnight and said a large group had "harassed and assaulted a visitor and a journalist".
Some protesters said they believed one of those men was an undercover Chinese agent, while another was confirmed as a reporter from China's Global Times newspaper.
Five people were detained in the latest disturbances, police said, bringing the number of those arrested since the protests began in June to more than 600.
The protests 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.
Demonstrators say they are fighting the erosion of the "one country, two systems" arrangement that enshrined some autonomy for Hong Kong when it returned to China in 1997.
In Washington, Trump said the Chinese government was moving troops to the border with Hong Kong and urged calm.
China's People's Liberation Army (PLA) has a garrison in Hong Kong but has remained in barracks since the protests started in April. However, the PLA garrison has issued a video showing "anti-riot" exercises, and its top brass have warned violence is "absolutely impermissible".
Hong Kong's Airport Authority said it would only allow entry for passengers with a boarding pass valid for the next 24 hours and had obtained an interim court injunction to stop people from obstructing operations. Protesters are only allowed to demonstrate in designated areas.
Some protesters expressed remorse for the clashes overnight. One 22-year-old frontliner who identified himself as Pun said protesters needed to re-evaluate their strategy to continue with the fight.
"We are not trying to overthrow the government or cut ourselves off from China. But we fight for our rights; democracy was promised as part of One country Two systems."
Forward Keys, a flight data company, said the crisis had deterred people from making travel plans to the city, citing a 4.7 per cent fall in long-haul bookings to Hong Kong between June 16 and August 9 compared with the same period last year.
Statements of apology from protesters were displayed in the airport on Wednesday, promising to allow passengers to depart, to assist medical staff to carry out their duties and not to hinder the work of the press.