China says Hong Kong's protest movement has reached "near terrorism", as more street clashes followed ugly scenes the previous day when protesters set upon men they suspected of being government sympathisers.
The US said it is deeply concerned at news of Chinese police forces gathering near the border, and urged Hong Kong's government to respect freedom of speech.
The US also issued a travel advisory urging caution when visiting the city.
By nightfall on Wednesday police and protesters were again facing off on the streets, with riot officers shooting tear gas almost immediately as their response to demonstrators toughens.
Operations at Hong Kong’s airport came to a halt after protesters occupied the main terminal. Source: AAP
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.
The chaos was sparked by protesters swarming the airport and, late on 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 
"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.
Police and protesters clash on Hong Kong's streets, complete with tear gas. Source: AAP
"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 the control of activists who then detained a Chinese reporter for a short time.
The protests have 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."
Protesters occupy the arrival hall of the Hong Kong International Airport during a demonstration on August 12, 2019. Source: Getty Images AsiaPac
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, with the People's Daily calling for "using the sword of the law" to restore order.
Mainland social media users lauded the detained reporter as a hero.
