Hong Kong’s airport canceled all flights Monday after thousands of pro-democracy protesters stormed into the main terminal of one of the world’s busiest travel hubs to denounce police violence.

“Airport operations at Hong Kong International Airport have been seriously disrupted … all flights have been canceled,” the city’s airport authority said in a statement. “All passengers are advised to leave the terminal buildings as soon as possible.”

Hong Kong has been roiled by mass protests calling for democratic reforms and an independent investigation into police conduct, with both the demonstrators and police turning to more extreme tactics.

In Beijing, the cabinet’s Hong Kong and Macao Affairs Office issued a statement saying the situation in the former British colony was “beginning to show the sprouts of terrorism” and constituted an “existential threat” to the population of Hong Kong.

“One must take resolute action toward this violent criminality, showing no leniency or mercy,” spokesman Yang Guang said in the statement.

“Hong Kong has reached an inflection point where all those who are concerned about Hong Kong’s future must say ‘no’ to lawbreakers and ‘no’ to those engaged in violence,” he added.

Earlier Monday, police showed off water cannons that could be deployed in the case of future demonstrations, a development that Amnesty International has warned could lead to serious injuries.

“Water cannons are not a toy for the Hong Kong police to deploy as a sign of strength,” Man-kei Tam, the group’s Hong Kong director, said in a statement.

“These are powerful weapons that are inherently indiscriminate and have the potential of causing serious injury and even death.”

The slogan “An eye for an eye” was plastered all over the airport — a reference to a female protester whose eye was injured during clashes with riot police who fired tear gas and beanbags on Sunday, according to CNN.

Protesters handed out lists to arriving visitors documenting alleged police violence.

“I just don’t understand how people can tolerate that kind of police brutality. I feel like if I don’t come out now, I can’t come out ever,” said Hilary Lo, an accounting firm worker, according to the Guardian.

“People are starting to realize the police are out of control, especially with what has happened in the past two weeks,” she added.

A police spokesman said there wasn’t enough evidence to determine the cause of the woman’s injury and that authorities won’t investigate unless someone files a report on the incident.

The Chinese-ruled territory faces its most serious crisis in decades, as Chinese leader Xi Jinping grapples with one of his largest popular challenges since he came to power in 2012.

The demonstrations began in opposition to a bill allowing extradition to the mainland but have widened to highlight other grievances.

Demonstrators say they are fighting the erosion of the “one country, two systems” arrangement that has provided some autonomy for Hong Kong since China took it back from Britain in 1997.

They are demanding the resignation of the city’s leader, Carrie Lam, and an independent probe into the handling of the protests.

With Post wires