Trump stoked fears of a Chinese intervention, saying in a tweet that reports from US intelligence agencies show mainland troops massing at the border with Hong Kong. Overwhelmed by the protesters, the police took out his pistol, which made the protesters disperse. He was later taken away by ambulance.
The action followed an unprecedented shutdown of the airport on Monday.
Many Hongkongers feared the law would be used by authorities to target political enemies and that it would signify the end of the "one country, two systems" policy, eroding the civil rights enjoyed by Hong Kong residents since the handover of sovereignty from the United Kingdom to China in 1997.
Forward Keys, a flight data company, said the crisis had deterred people from making travel plans to the city, citing a 4.7 percent fall in long-haul bookings to Hong Kong between June 16 and August 9 compared with the same period past year. Ten weeks of increasingly violent clashes between police and protesters have roiled the Asian financial hub as thousands of residents chafe at a perceived erosion of freedoms and autonomy under Chinese rule.
Sean Lavin, an American who is in Hong Kong on vacation, said that he and his travel companions are slated to fly out of the airport on Wednesday, but the protests have left that in question. China rejected it, calling it a "wrongful statement" amounting to interference in its domestic affairs and sent "the wrong signal to violent criminal offenders".
Hong Kong protesters have apologised to the public for the chaos caused at the city's airport, one of the world's busiest transport hubs, after demonstrations brought flights to a halt for two consecutive days and stranded thousands of visitors.
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They want Lam to resign. They present President Xi Jinping with one of his biggest challenges since he came to power in 2012.
Hong Kong legal experts say Beijing might be paving the way to use anti-terrorism laws to try to quell the demonstrations.
Earlier in the day, authorities were forced to cancel all remaining flights as the city's pro-Beijing leader warned that the protesters had pushed events onto a "path of no return".
Britain, a guarantor of the agreement that transferred Hong Kong to China in 1997, on Tuesday condemned the violence and urged dialogue. The protesters that were thought to be working for police were held, beaten and brutalized by the angry group of activists according to reporting by the Associated Press.
"It was something I've never experienced before", he said, adding that the protesters were "very polite" and helped his group find their way out of the airport.
It said it did not expect arriving flights to be affected, though dozens were already cancelled. The airline, whose British heritage makes it a symbol of Hong Kong's colonial past, is also in a political bind.
China's aviation regulator demanded last week that Cathay suspend personnel who engaged in or supported protests in Hong Kong from staffing flights into its airspace.