HONG KONG — Pro-democracy demonstrators in Hong Kong began a new campaign on Sunday to squeeze access to the airport, hours after one of the most tumultuous days since protests in the city began in June.
Tens of thousands of people marched through parts of the city center on Saturday despite a ban on the protest by the police. Some protesters gathered around the local government’s headquarters, where they threw bricks and firebombs as the police responded with tear gas, pepper spray and water cannons.
On Sunday, demonstrators began congregating at Hong Kong International Airport in a new effort to obstruct access to the critical Asian travel hub. The airport has been off limits to protesters since mid-August, when days of sit-ins led to assaults on two men from mainland China and hundreds of canceled flights.
With classes set to begin on Monday for many Hong Kong students, schools could become the next front in the protest movement, which began over widespread anger over an extradition bill that would allow criminal suspects to be taken to mainland China. Since then, demonstrators’ demands have grown to include a call for universal suffrage and an inquiry into accusations of police brutality.
Hundreds of protesters began to converge on the airport Sunday afternoon, traveling by bus, by car and on foot from a nearby subway station. A court injunction obtained after the airport protests last month allows only ticketed passengers and airport employees to enter the main terminals. But demonstrators gathered outside near the entrances, chanting, “Fight for freedom! Stand with Hong Kong.” Some used their cars to block lanes of traffic.
“We have been protesting and occupying for months,” said Daniel Chan, 18, a college student who took a bus to the airport protest. “Still, what we have done seems futile.”
Mr. Chan said he did not intend to do anything illegal, but he was not concerned about the court injunction. “There’s almost nothing left for me to be scared of,” he said. “One document cannot deter me.”
MTR, the Hong Kong subway operator, announced on Sunday afternoon that the Airport Express rail service between the airport and the city center was canceled, and trains to the city were later suspended after protesters threw debris on the tracks. Tung Chung Station, the subway stop closest to the airport, was also closed on Sunday evening because protesters had damaged the facilities, MTR said.
Ms. Zhao, who is from mainland China and works in education, had just landed in Hong Kong from Beijing when she received a notice from the airline suggesting that she postpone her trip.
“What the protesters are doing is crazy,” she said. “This just shows how different the Hong Kong and mainland systems are.”
Other travelers were more supportive of the protests.
Eric Jabal, 47, an education consultant, walked with his roller suitcase and suit jacket on a debris-strewn road, dodging protesters and barricades. He said he had left his home at 12:30 p.m. for an 8:50 p.m. flight to Bangalore and had been walking for 25 minutes after he had been dropped off by an Uber.
But Mr. Jabal, a Canadian who has been living in Hong Kong for 25 years, said he didn’t mind the inconvenience.
“I’m really sad,” he said. “That the failure of leadership has led to such profound unrest among such a broad cross section of people — it’s clearly gone beyond the tipping point.”
Students have been a major part of the protests all summer, and the beginning of classes on Monday raised questions about whether the start of school would mean a lessening of the movement or whether activism would shift to campuses.
Students have planned two mass assemblies, one in the central business district in the morning and another after school at the Chinese University of Hong Kong. Many high school students have also negotiated with school administrators to participate in sit-ins or to set aside classrooms for self-study sessions or silent protests on the first day of school.
Education authorities said that students would need parent letters to skip class or to participate in strike activities.
A top government official said on Sunday that the administration “steadfastly opposed” the planned class boycotts, calling them extremely irresponsible.
“Schools are places for learning, and are absolutely not places for expressing political views or demands,” Chief Secretary Matthew Cheung wrote in a blog post.
Earlier Sunday, the Hong Kong police said that they had arrested dozens of people in a subway station that was the scene of some of the most intense violence of the weekend, where officers used clubs and pepper spray on a group of protesters inside a train car.
News footage and videos shared on social media showed members of the police Special Tactical Squad, in dark uniforms with their faces covered, charging into a waiting train car at Prince Edward Station and swinging batons at men and women who cowered on the floor, offering no resistance. After a fury of blows, one officer doused the group in pepper spray and then left.
“The police were out of control,” said Crystal Yip, a 20-year-old college student who was in the station when officers arrived. “They were crazy and they were mad. They were trying to express their anger by attacking people randomly.”
Yolanda Yu, police senior superintendent, said 40 people were arrested in the station on suspicion of unlawful assembly, criminal damages and obstructing officers.
“Protesters used sticks and hard objects to attack police. We used the same level of force to respond to the situation,” Ms. Yu said, replying to a reporter asking why the police had used pepper spray on commuters who were kneeling on the ground.
She said that the police had warned civilians to stay away. “Under chaotic situations, it is indeed hard to determine whether someone is a real journalist, a protester or a violent person,” she said.
The violence in Prince Edward Station began during a dispute between protesters and some older men who were insulting them. One of the men swung a hammer at the protesters, who threw water bottles and umbrellas and later appeared to set off fire extinguishers in the car. After the clashes, the subway system suspended service across much of Hong Kong. Three stations remained closed on Sunday.
The subway operator, MTR, has been a target of vandalism since it began suspending service last month to stations in places near where protests are planned. It continued that pattern on Saturday, stopping service at Sai Ying Pun Station, near the Chinese government liaison office, a site of some protests.
In the wake of the clashes, Chinese news outlets run by the Communist Party urged the Hong Kong government to take tough steps against the protesters and cited experts urging Carrie Lam, the chief executive of the city, to invoke emergency powers. An online outlet controlled by the Communist Party’s law-and-order committee said the Hong Kong protesters were using “terrorist methods.”
“The chaos in Hong Kong cannot go on!” a front-page editorial in the overseas edition of People’s Daily, the party’s main newspaper, said on Sunday. “At this crucial moment, the government of the special administrative region must have the courage and adeptness to apply every legal means to halt the violence and chaos, acting resolutely to detain and arrest violent lawbreakers, applying the law strictly to punish criminals, and swiftly restoring social order.”
An online report from China’s main television broadcaster, CCTV, said that Ms. Lam should invoke emergency powers under a colonial-era ordinance to extinguish the violent protests. That step could empower the Hong Kong government to ban demonstrators from wearing masks, speed up arrests and censor “harmful media,” the report said.