Hong Kong descended further into chaos Tuesday as riot police clashed with protesters who had shut down the city’s airport for a second day and China positioned troops at the border of the region.

President Trump went on Twitter to announce the move by the Chinese military, which was seen massing its forces in the province of Shenzhen in an apparent threat to demonstrators who are decrying new laws they believe curb civil rights.

“Our Intelligence has informed us that the Chinese Government is moving troops to the Border with Hong Kong. Everyone should be calm and safe!” the president tweeted from Air Force One.

The demonstrations recall the 1989 Tiananmen Square protests for Chinese democracy, which Beijing violently suppressed, killing hundreds if not thousands.

The explosion of violence and anger at the airport is the high point of some 10 weeks of protests in the semiautonomous city-state.

For the past two days, protesters have kept the transit hub shut down. On Tuesday, they upped the ante by tying up a Chinese journalist, manhandling a person they suspected of being a Chinese cop and facing off with local riot police.

At one point, they even tried to stop authorities by using airport luggage carts to create a defense wall.

Carrie Lam, the territory’s chief executive, warned that the violence was pushing Hong Kong in a dangerous direction.

“Violence, no matter if it’s using violence or condoning violence, will . . . plunge Hong Kong society into a very worrying and dangerous situation,” she said.

A US official told CNN that the administration believes that if China intervenes militarily in the protests, it would likely be because Beijing had determined that Hong Kong authorities have lost control.

Protesters turned to vigilantism Tuesday and swarmed around a man they suspected of being a plainclothes cop from mainland China — using zip ties to cuff his hands, shouting insults, kicking and threatening to kill him.

A paramedic reached the man, who collapsed several times, but the crowd refused to let the emergency worker remove him.

“All consequences are at your own risk!” the protesters yelled, echoing the dire warning a People’s Liberation Army officer had uttered in an ominous propaganda video last month.

A group of about two dozen cops entered the airport at 11 p.m. to get the man out amid screams of “Gangsters! Gangsters!” from the protesters, some of whom attacked police vehicles.

Later, the crowds encircled another man who’d been snapping pictures with his cellphone, forcing him to sit on a luggage cart, tying his ankles and wrists, and shining lasers in his eyes.

They pulled an “I ♥ HK Police” T-shirt and mainland Chinese passport out of his bag, shouting that he was “fake press” and likely an undercover officer.

The man — who was eventually carried off by a paramedic — was later identified by the top editor of The Global Times of China as Fu Guohao, one of its employees.

“He has no other task except for reporting,” Hu Xijin, editor-in-chief of the state-run tabloid, said in a tweet. “I sincerely ask the demonstrators to release him. I also ask for help of West reporters.”

Meanwhile, The Global Times and the People’s Daily ran a minute-long video of clips of armored-personnel carriers purportedly en route to Shenzhen, which borders the semiautonomous territory.

“The Hong Kong thing is a very tough situation — very tough,” Trump told reporters earlier during a visit to Morristown, NJ. “We’ll see what happens.”

He added, “I think it will work out and I hope it works out, for liberty. I hope it works out for everybody, including China. I hope it works out peacefully. I hope nobody gets hurt. I hope nobody gets killed.”

US Secretary of State Mike Pompeo and top Chinese diplomat Yang Jiechi “had an extended exchange of views on US-China relations” on Tuesday, the State Department said without elaboration.

The United Nations’ top human rights official condemned the violence and called on both sides to solve the dispute peacefully.

A spokeswoman for the High Commissioner for Human Rights, Michelle Bachelet, said her office had also reviewed credible evidence that police were using “less-lethal weapons in ways that are prohibited by international norms.”

China rejected her “wrongful statement,” saying it amounted to interference in its domestic affairs and sent “the wrong signal to violent criminal offenders.”

With Post wires