Hong Kong airport authorities canceled remaining flights on Monday after protesters swarmed the main terminal building for a fourth day, the biggest disruption yet to the city's economy since demonstrations began in early June.

Thousands of black-clad protesters on Monday packed the arrival area, where they had gathered for a three-day sit-in that was originally planned to end last night. The protests, initially sparked by opposition to a bill that would allow extraditions to mainland China, have become increasingly violent in recent weeks, with demonstrators targeting public transportation in a bid to pressure the government.

It was unclear how many flights were impacted, according to Doris Lai, a spokesperson for the Hong Kong Airport Authority. The airport said in an earlier statement that it was aiming to restore operations as soon as possible after canceling all flights for the rest of the day, except those already in the air.

Shares of Cathay Pacific Airways Ltd., Hong Kong's main airline, tumbled to a 10-year low after the news. The government planned a press briefing for 5:15 p.m. local time. The Stoxx Europe 600 Index came off its session high and contracts for all three main U.S. equity indexes erased earlier gains.

China stepped up its rhetoric on Monday, saying protesters have committed serious crimes and showed signs of "terrorism." Hong Kong has come to a "critical juncture" and all people who care about its future should say no to violence, Yang Guang, a spokesman for its Hong Kong and Macau Affairs Office, told reporters on Monday as protesters gathered at the airport.

"All those who care about Hong Kong's future should come out and stand against all criminal acts and perpetrators of violence," Yang told reporters.

The protest have evolved into the biggest challenge to Chinese control since the U.K. relinquished its former colony in 1997. The social unrest has hurt the economy and impacted daily life in one of the world's most densely crowded cities, raising concern that Beijing will use force to restore order.

Stoking those fears, the Communist Party-backed Global Times reported on Monday that the Chinese People's Armed Police have been assembling across the border in Shenzhen ahead of "apparent large-scale exercises."

Tang Ping-keung, deputy commissioner of police, said it was too early to say whether force would be used to clear the airport. "It will be up to commander to decide" whether to use tear gas, he told reporters.

Authorities had deployed more aggressive tactics during the weekend protests, with riot police videotaped beating demonstrators in subway stations and officers going undercover to infiltrate the group and make arrests. The violent scenes emerged as protesters used flash mobs across the city, surrounding police stations, disrupting traffic, and hurling projectiles including bricks and petrol bombs. One officer was taken to the hospital after suffering burns in the upmarket shopping district of Tsim Sha Tsui. Mob violence broke out elsewhere.

Police responded with tear gas and rubber bullets at various locations -- including inside a metro station for the first time. Dramatic videos showed riot police firing weapons at close range and beating some protesters, many of whom wore yellow hard hats and gas masks. Some 13 protesters were injured, including two in serious condition, RTHK reported, citing hospital authorities.

Cathay Pacific has come under fire after some of its employees joined the demonstrations. A Chinese state-run company told employees not to fly Cathay Pacific on business or personal trips, according to people familiar with the matter.

Hong Kong leader Carrie Lam has refused to yield to a series of demands, including that she withdraw the bill and step down from her position. Authorities in Beijing remain supportive of her government, which has warned of an economic crisis if the demonstrations drag on.

The protesters are resorting to flash mobs and violence as their numbers diminish, according to Steve Vickers, chief executive officer of risk consultancy Steve Vickers and Associates and a former head of the Royal Hong Kong Police Criminal Intelligence Bureau.

"The government's policy of sitting on their hands and hiding behind the police is actually working," Vickers told Bloomberg Television on Monday. "The numbers are declining, the level of violence is increasing. As violence increases, the more middle class people and ordinary people of Hong Kong will turn against this movement."

China in recent weeks has toughened its stance toward the movement and doubled down on its support for the police. The Hong Kong and Macau Affairs Office, its top agency overseeing the former British colony's affairs, has held unprecedented briefings condemning violent protesters and called on the people of Hong Kong to oppose them. An overseas edition of the Communist Party mouthpiece People's Daily said last month that police should take stern action to restore order.

Hong Kong called former deputy police commissioner Lau Yip-shing out of retirement last week to handle major upcoming public events including celebrations marking the 70th anniversary of the founding of the People's Republic of China in October. Lau had overseen the government's crackdown on protesters during the 2014 pro-democracy Occupy movement.

Authorities had denied permits over the weekend for protests in all but Victoria Park, but demonstrators took to the streets anyway. Police made more arrests on Sunday after detaining 16 people on Saturday, with local media reporting that officers may be dressing as protesters and infiltrating their ranks to help with detentions.

China's civil aviation authority had earlier told Cathay Pacific to ban all employees who supported or joined the recent protests from flying to the mainland, one of the strongest signs yet that Beijing is losing its patience with the demonstrations.

Cathay suspended a pilot from flying who had been detained while participating in a protest, the airline said in a statement. It also fired two workers for "misconduct." They allegedly leaked information about the travel arrangements of a Hong Kong police soccer team, the South China Morning Post reported.

"As always our actions and responsibilities are focused on the safety and security of our operations," the airline said.

This weekend's protests come days after a general strike that disrupted the financial hub's morning rush hour, leaving traffic jammed, subway lines suspended and dozens of flights canceled. Those demonstrations also ended in tear gas and dispersal operations.

"It's affected business tremendously -- all businesses basically," Allan Zeman, chairman of Hong Kong's Lan Kwai Fong Group, which operates restaurants and bars in the city, told Bloomberg Television. "We have to stop the violence. That's the most important thing. Then we can talk."

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Bloomberg’s Fion Li, Will Davies and Justin Chin contributed.