Hong Kong is bracing for more mass demonstrations through the weekend, with the crisis escalating after pro-democracy protests forced the cancellation of nearly 1000 flights this week and world leaders urging calm.
China reiterated on Wednesday that Hong Kong's protest resembled terrorism and more street clashes followed chaotic scenes at the airport two days ago, when protesters set upon two men they suspected of being government sympathisers.
Police and protesters faced off again on the streets of the financial hub overnight, with riot officers quickly firing tear gas as their response to demonstrators hardens.
Ten weeks of increasingly violent confrontations between police and protesters have plunged Hong Kong into its worst crisis since it reverted from British to Chinese rule in 1997.
US President Donald Trump tied a trade agreement with China to the protests being resolved "humanely", even suggesting that he was willing to meet Chinese President Xi Jinping to discuss the crisis.
"I have ZERO doubt that if President Xi (Jinping) wants to quickly and humanely solve the Hong Kong problem, he can do it. Personal meeting?" Trump said on Twitter.
The US State Department said earlier it was deeply concerned about reports that Chinese police forces were gathering near the border with Hong Kong and urged the city's government to respect freedom of speech.
A satellite photo showed military vehicles massed inside a sports stadium in the Chinese city of Shenzhen, which borders Hong Kong.
The State Department also issued a travel advisory urging US citizens to exercise caution when visiting Hong Kong.
French Foreign Minister Jean-Yves Le Drian called on Hong Kong authorities to renew talks with protesters to find a peaceful solution, while Canadian Prime Minister Justin Trudeau urged China to handle the protests with tact.
The Australian Chamber of Commerce in Hong Kong also urged all parties to engage in constructive talks to restore the city's standing as an international business hub.
Hong Kong's Court of Appeal granted bail on Thursday to law professor Benny Tai, one of the movement's leaders who had been in jail for two months for public nuisance offences relating to the "Occupy" protests.
Tai said he was very touched by the sacrifices people had made to defend the city's core values. "The golden era of Hong Kong is yet to come, and I believe the time is not too far away," said Tai, whose appeal remains pending.
Hong Kong's Airport Authority said normal flight operations would resume on Thursday but heightened security would remain at the city's international airport. It said on Wednesday an application for protests to be held in the terminal must be made in advance with a "Letter of No Objection" from police.
Protesters have expressed remorse after a peaceful sit-in turned violent at one of the world's busiest airports earlier this week.
Research firm Capital Economics said the protests could push Hong Kong into a recession, with a growing risk of "an even worse outcome if a further escalation triggers capital flight".
The Chinese Securities Association of Hong Kong said the city's reputation would be seriously damaged if the violence and unrest were not stopped as soon as possible.
Protests were planned across different districts of Hong Kong from Thursday, including a teachers rally, and one organised by animal lovers upset that their pets were being tear-gassed.
The protests grew out of opposition to an extradition bill into wider concerns about the erosion of freedoms guaranteed under the "one country, two systems" formula put in place after the return to Chinese rule in 1997.