Hong Kong International Airport, one of the world's busiest terminals, has canceled all departures for the remainder of the day, citing serious disruption due to pro-democracy demonstrations. The airport authority said Monday it had canceled all flights not yet checked in by the afternoon. Around 5,000 anti-government protesters had been demonstrating at the airport for a fourth day on Monday. Some activists had reportedly moved to the departure area and caused disruption, according to the Hong Kong police. The police declined to say if it would move to clear the anti-government demonstrators. The authority said in a statement: "Airport operations at Hong Kong International Airport have been seriously disrupted as a result of the public assembly at the airport today." "The traffic to the airport is very congested, and the car park spaces at all car parks are already full. Members of the public are advised not to come to the airport," it added. It later advised all passengers to leave the terminal building as soon as possible.

It said flights will resume at 6 a.m. Tuesday local time (6 p.m. Monday ET).

Two months of protests

The increasingly violent protests since June have plunged the Asian financial hub into its most serious crisis in decades and are one of the biggest popular challenges to Chinese leader Xi Jinping since he came to power in 2012.

Despite the intense police response and a toughening stance from China, the movement that began more than two months ago in opposition to a bill allowing extradition to the mainland still seems to enjoy broad support in the city of more than 7 million people. The unrest in Hong Kong — a former British colony that was returned to Chinese rule in 1997 — has frequently crippled its transportation system. The rallies have snowballed into a democracy movement, with some even demanding full autonomy from Beijing. A senior official in the Trump administration said in a statement to CNBC that the U.S. is monitoring the situation in Hong Kong and "societies are best served when diverse political views are respected and can be freely and peacefully expressed." Hong Kong-carrier Cathay Pacific advised customers to postpone nonessential travel and said passengers should not proceed to the airport. "While disruption events like these can change significantly and at short notice, rest assured we are doing everything we can in advance to minimize the impact to customers," the airline said in a statement. It added that the cancellation period for Hong Kong flights will last until Tuesday morning, according to Reuters.

Eighth-busiest airport

Scores of protesters were arrested over the weekend across the city, with a reported 600 in total being detained since the unrest began. A Chinese official in Beijing reportedly claimed that signs of "terrorism" were emerging from the protests. "Radical Hong Kong protesters have repeatedly used extremely dangerous tools to attack police officers," a spokesman for the Chinese government's Hong Kong and Macau Affairs Office told a briefing Monday, according to Chinese state media. "The first signs of terrorism are starting to appear." Hong Kong International 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.

Protesters occupy the departure hall of the Hong Kong International Airport during a demonstration on August 12 in Hong Kong. Anthony Kwan | Getty Images