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.

The protests, sparked in June by a bill easing extraditions to the mainland, 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.

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.

Read an explainer on Hong Kong’s protest movement

Addressing the closing ceremony of a Chinese army training camp for college students on Sunday, Lam vowed to do a better job of connecting with the young people of Hong Kong who have been the main drivers of the protests, the South China Morning Post reported. Authorities “will be more patient to get in touch with youth from different classes to listen to their voices,” Lam said, speaking in Mandarin.

The protesters are resorting to flash mobs and violence as their numbers decline, which will diminish support for the movement among the broader public in the coming months, 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.

“Last night’s pictures are pretty ugly, but this morning the city is functioning as if nothing had happened,” he said. “The stock exchange is open, traffic is running, everybody’s gone back to work. So it’s easy to see these pictures and form perhaps a wrong impression of what’s going on here.”

Protesters and bystanders have also in recent weeks found themselves targeted by gangs of unidentified men, including a July 21 attack in the northern suburb of Yuen Long that resulted in the arrest of some suspects with connections to the city’s triad gangs. The attacks have raised fears that thug violence could spread and add a new dimension to the growing unrest.

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.

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.

Read more: China cracks down on Cathay staff joining protests

By Sunday night a three-day sit in at Hong Kong’s international airport was winding down. Thousands of demonstrators had greeted passengers with “Free Hong Kong” chants. Only departing passengers with tickets or boarding passes and valid travel documents were being allowed to enter the check-in area at Terminal 1.

China’s civil aviation authority told Cathay Pacific Airways Ltd., Hong Kong’s main airline, 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 shares slid as much as 4.7% on Monday, their biggest loss since Oct. 25.

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.