Later, President Donald Trump tweeted that U.S. intelligence informed him that Chinese troops were being moved to the Hong Kong border.
Meanwhile, the city's embattled leader, Carrie Lam, told the news media on Tuesday that "lawbreaking activities in the name of freedom" were damaging the rule of law and that the Asian financial hub's recovery from anti-government protests could take a long time.
Her comments came after Beijing said widespread anti-government protests in the semi-autonomous city showed "sprouts of terrorism," and such violence must be severely punished, "without leniency, without mercy."
The Hong Kong Airport Authority said that airport operations were "seriously disrupted" by a large rally of protesters whom the riot police held back with pepper spray. Protesters blocked passengers from continuing to immigration checkpoints, and some police cars were blocked during heated encounters with police.
Following scuffles between protesters and police, an injured person was taken away from the main terminal by medics.
Hong Kong's airport had reopened earlier after airport authorities canceled all flights on Monday, blaming demonstrators' disruption to regular operations.
Despite that reopening, Hong Kong flag carrier Cathay Pacific said it had cancelled over 200 flights to and out of the airport for the day, according to its website.
On Monday, Chinese officials focused on what they described as "deranged acts" by the protesters, including throwing gasoline bombs, saying they marked the emergence of terrorism in the Chinese city.
"Radical Hong Kong protesters have repeatedly used extremely dangerous tools to attack police officers," Yang Guang, a spokesman for the Chinese government's Hong Kong and Macau Affairs Office, said in a news briefing on Monday, according to Chinese state broadcaster CCTV.
China's media is sending a clear signal to the protesters.
On Monday afternoon, Chinese state-owned English tabloid the Global Times tweeted a video showing the People's Armed Police assembling in Shenzhen, a city bordering Hong Kong, about a 1.5 hour drive away.
The People's Daily, the official newspaper of China's Communist Party, posted on Chinese social media a statement saying the People's Armed Police are in Shenzhen prepared to handle "riots, disturbance, major violence and crime and terrorism-related social security issues."
In a Tuesday social media post from the Global Times‘ Chinese edition, the outlet said "if Hong Kong rioters cannot read the signal of having armed police gathering in Shenzhen, then they are asking for self-destruction," according to a CNBC translation.
China is "implying they might send in the People's Liberation Army or issue direct intervention but they don't want to," according to Ben Bland, a director at Sydney-based policy think tank Lowy Institute.
"(Beijing) hopes that the signals will scare protesters to back down," but if and when Beijing decides to deploy troops they will not "advertise it," he told CNBC. This is all part of a "delicate dance between China and Hong Kong" that's reached a critical point because there is almost no common ground or overlapping interests between the protesters and Beijing, Bland added.
Although China's leaders do not want to deploy the PLA, they are "willing to do it if they have to," the Asia politics expert said.
Hong Kong's former governor, Chris Patten, said on Tuesday that if China intervened in the city, it would be a "catastrophe" and that Chinese President Xi Jinping should see the wisdom of trying to bring people together.
Patten called on British Prime Minister Boris Johnson to garner support from its allies to ensure Beijing does not intervene.
"I hope nobody gets hurt. I hope nobody gets killed," Trump told reporters on Tuesday after he was asked about the Hong Kong protests.
The U.N. High Commissioner for Human Rights released a statement on Tuesday, expressing concern about the "escalation in violence" and encouraging demonstrators to act with restraint.
However, China rejected what it called a "wrongful statement" by the U.N. and said that it was sending "the wrong signal to violent criminal offenders". China's mission to the U.N. in Geneva called out the protesters' destruction of public facilities, blocking of public transportation, and use of lethal weapons, "showing a tendency of resorting to terrorism".
"The Chinese central government firmly supports Hong Kong Chief Executive Carrie Lam and the HK SAR government in discharging their duties in compliance with the law and supports the Hong Kong police force and judicial organs in enforcing the law decisively," the statement said.
An EU spokesperson called for both sides of the conflict to demonstrate restraint and "de-escalate the situation."
"It is now more vital than ever to engage in a political process of broad-based and inclusive dialogue, involving all key stakeholders."
On Monday Hong Kong's airport halted all outbound flights, citing disruptions from a sit-in by protesters that had begun on Friday morning. The ongoing demonstrations in Hong Kong — a former British colony that was returned to Chinese rule in 1997 — have frequently hobbled its transportation system. What started as peaceful rallies against a single proposed law have snowballed into a wider pro-democracy movement, with some even demanding full autonomy from Beijing and occasional outbreaks of violence.
According to Bland, it has become more apparent in the last few weeks that Hong Kong's government is "increasingly only operating on direct instruction or consultation with Beijing," and acting more like a mainland Chinese regional government since Lam's push to enact a law allowing extraditions to the mainland — the original impetus for the protests.
Many Hong Kongers reject such coordination on the basis of the "one country, two systems" principle that was promised to the city after the handover from the British. That concept was proposed when the former British colony was reunited with the mainland, and guarantees that the city would maintain a separate economic and legal system.
In essence, the ongoing movement is a fight to protect Hong Kongers' "political identity" as much as it is specifically focused on furthering democracy in the city, Bland said, explaining that it constitutes a demand for self governance and a fairer economy.
For now, the city is bracing for more protests. Hong Kong International Airport is one of the world's busiest transportation hubs in the world, it is the eighth-busiest airport in the world and handles over 72 million passengers a year, according to the latest statistics by Airports Council International.
— Reuters and CNBC's Matt Clinch contributed to this report.