Hard-line demonstrators and the embattled authorities are locked in an impasse.

A core group of combative young agitators has come to define the antigovernment protests that have convulsed Hong Kong since June.

HONG KONG — Fat Boy is a college dropout with a youthful blush of acne who excels at playing video games and lives with his mother. He is also a wily commander who leads a ragtag band of protesters willing to risk injury and arrest as they face off against the police.

Fat Boy oversees 50 or so Hong Kong protesters, ages 15 to 35, who focus their attacks on the police, government offices and Chinese-owned banks or other businesses they view as hostile to their movement. Their weapons — bricks, poles and Molotov cocktails — are often met with tear gas and rubber bullets. Occasionally, the police have responded with live fire.

They are part of a core of combative young agitators, garbed in black, who have come to define the antigovernment protests that have convulsed this semiautonomous territory for more than four months and that have posed a bold challenge to the authority of China’s ruling Communist Party.

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“You have to earn your rights and freedom,” Fat Boy, 20, said one afternoon this month at his apartment as he and three team members picked at takeout food and talked about their anxieties and aspirations. “For this, we can have no regrets.”

Image The protest leader known as Fat Boy in Hong Kong on Saturday.

With a self-confidence that frequently veered to bravado, he showed off photos from the day he hot-wired an excavator at a construction site and drove it to the entrance of a police station.

“After I did that, other people copied me,” he said.

The protesters have escalated their use of violent tactics, smashing storefronts, setting bonfires at subway stations and taking justice into their own hands.

On Oct. 13, a protester stabbed a police officer in the throat with a box cutter, leaving him in serious condition. The same day, a homemade bomb detonated by cellphone exploded in a sidewalk planter, though it caused no damage or injuries.

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The police warned that the political movement had entered a dangerous new phase.

Image Parts of a homemade bomb that exploded on Oct. 13.

Hard-line protesters and the authorities are locked in an impasse that feels as if it is edging closer to a fatality or perhaps even an intervention by Chinese troops that could further endanger the civil liberties long enjoyed by the territory’s seven million residents.

The protesters say they have been driven to extremes by a government that won’t meet their demands for greater democracy and an investigation into police conduct. The authorities refuse partly because they believe further concessions would only encourage more violence.

Francis Lee, a journalism professor at the Chinese University of Hong Kong who has conducted public opinion surveys on the unrest, said the police and protesters were trapped in a vicious cycle of surging violence.

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“We have reached a stage where it is difficult for either side to escalate their actions without creating backlash,” he said.

The city’s leaders and the police, seeking to drive a wedge between the front-line protesters and the broader public, have cast the demonstrators as rioters and violent vigilantes. Moderates who support the movement’s goals express a growing fear that the violence plays into Beijing’s hands and will undermine public support.

Image Protesters clashing with police officers in riot gear in the Sha Tin area of Hong Kong on July 14.

For now, Hong Kong’s residents continue to back the protests by wide margins despite the violence, said Professor Lee, whose polling suggests that at least creating an independent commission on police conduct would go a long way toward defusing the crisis.

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“These demands are not radical at all,” he said. “Doing them is very likely to be adequate to kick-start a process of de-escalation.”

Fat Boy, his three teammates and one other front-line protester agreed to be interviewed on the condition that they be identified by nicknames or first names only, for fear of being arrested.

The teammates include Jeff, a 24-year-old musician and skateboarder who quit his full-time job renovating apartments to devote himself to the protest movement; Kitty, a 21-year-old English major who recently left school for the same reason; and Tyler, 34, a construction manager who supports the brigade by supplying the helmets and carbon fiber shields that protect its members during confrontations with the police.

They said they wanted to push back against the Hong Kong government’s narrative likening them to thugs and mainland China’s propaganda that describes them as separatists. Their goals: to seek police accountability and secure the universal suffrage that they say Beijing promised when this former British colony was returned to China in 1997.

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Like many Hong Kong youths, the front-line protesters assert an identity that, unlike in generations past, is fiercely distinct from that of mainland China.

Their anger is rooted in a growing sense that China’s Communist Party has worked swiftly to erode Hong Kong’s civil liberties, and as examples they point to the ousting of opposition lawmakers and detention of city booksellers by the mainland authorities.

Image With their gas masks and ninjalike attire, the front-liners are an unmistakable presence during demonstrations.

Samuel, 24, a freckle-faced protester and aspiring songwriter who is not part of Fat Boy’s group, explained, “We just don’t want to become like those Chinese who have become accustomed to living without freedom.”

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During protests, Samuel erects roadblocks to slow down advancing columns of officers. He and the others defended their tactics and said they were being driven by mounting police brutality and an inflexible government.

A few of the democracy advocates conceded that the homemade bomb gave them pause.

Longhaired, lanky and contemplative, Jeff, the musician, equivocated when asked whether he approved of the bombing. He said that he didn’t think he could use such a weapon but that he might reconsider if its sole aim was to sow chaos and disperse charging police officers.

“I used to be someone who wouldn’t even throw a brick,” he said. “But every time I encounter an escalation by the police, my limit goes higher.”

Image A protester with a shield made from a street sign.

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With their gas masks and sleek, ninjalike attire, the front-liners are an unmistakable presence during demonstrations. They have adopted the martial arts hero Bruce Lee’s ruminations on flexibility in the face of obstacles — “Be water, my friend” — saying they should behave like a wave that appears at once to pound the enemy and then promptly recedes into countless drops that cannot be contained.

They coordinate moves on encrypted messaging apps and are aided by four tacticians on three continents who remotely monitor the street battles.

It is impossible to know the number of antigovernment protesters who have embraced a more violent approach, but Fat Boy says he is aware of as many as 30 groups whose leaders meet face to face once or twice a month. The groups operate autonomously, with infrequent contact, an arrangement that helps protect them from arrest.

Fat Boy and those who direct the activities of the other groups are the closest thing to commanders in a movement largely characterized by the absence of readily identifiable leaders.

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“It would be dangerous to talk to each other,” he said. “If one gets caught, all of us get caught.”

Image Hong Kong police officers arrest protesters during a demonstration in September. Credit... Lam Yik Fei for The New York Times

So far, the police have arrested around 2,700 people, though most have been released on bail. About 200 could face 10 years in prison on charges of rioting. Roughly a third of them are under 18, and 100 of them are under 16.

Many of Fat Boy’s claims could not be independently verified, but his mother and the three front-liners on his team corroborated much of what he said.

He has named his team Hogwarts, after the mythical school of wizardry in the Harry Potter series, and says his group was one of the first to use Molotov cocktails, a weapon that was quickly embraced by other front-line protesters as a way to slow the advance of the riot police.

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His team has been especially busy this fall. On Sept. 29, two days before National Day celebrations in China, several team members set fire to a subway station entrance, producing huge plumes of black smoke and drawing a battalion of firefighters.

On Oct. 1, they donned Guy Fawkes masks, burned paper portraits of China’s president, Xi Jinping, near military barracks and quickly vanished.

Image Protesters set fire to a subway station entrance in Central Hong Kong on Sept. 29.

At first glance, Fat Boy is hardly menacing. He shuffles around his mother’s luxury apartment in slippers, toggling between cable news channels and video games like Red Dead Redemption. He rarely sleeps more than a few hours each night.

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“I’m mentally and physically exhausted,” he said. “I think the police are tired too.”

As he spoke, Fat Boy’s mother cleaned up the apartment while Jeff and Kitty cuddled on the sofa — their romance began at a protest this summer.

Fat Boy’s mother was once largely apolitical but now attends the protests, handing out food and tending to those overcome by pepper spray or tear gas.

“Every time he goes out, I worry he and his friends will get hurt, or that they might not come home at all,” she said, wringing her hands.

Fat Boy said that his schooling influenced his political awakening. He attended an elite Hong Kong academy that he described as pro-China. It sent him and his classmates to the mainland for a week of military training, where he learned the value of leadership and how to fire a gun.

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He later went to school in Canada, where he gained an appreciation for Western-style democracy and civil liberties. He is also well versed in military history and especially World War II, which he says taught him the importance of standing up to tyranny.

Fat Boy’s evolution from peaceful demonstrator to firebomb-throwing provocateur mirrors that of other combative protesters.

On June 9, he joined a million people who took to the streets in the very first protest against a contentious bill that would have allowed residents to be extradited to mainland China. Fat Boy and many others were infuriated after the city’s leader, Carrie Lam, said the legislation would proceed as planned, driving home the idea that peaceful demonstrations were useless.

Image On June 9, a million people took to the streets in the first protest against an extradition bill that was eventually withdrawn. Credit... Lam Yik Fei for The New York Times

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Three days later, on June 12, he joined thousands of protesters in surrounding the legislative complex to block debate about the bill. A core group lobbed umbrellas and bricks at the police, who responded with what critics say was excessive force, beating protesters with batons and dousing them with pepper spray and plumes of tear gas.

“On June 9, I was just a guy passing out fliers on the street. By June 12, I had 100 people at my side,” he said, referring to the confrontational core of protesters.

Image The protests escalated on June 12, when protesters surrounded Hong Kong’s legislative building.

To prevent infiltration by undercover officers, Fat Boy scrutinizes the social media accounts of new recruits. So far, he said, only five or six of his members have been arrested, all of them during street fights with the police.

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Their support network includes Hong Kongers in Canada, Australia and Britain who help coordinate attacks on the police, he said. Studying protester chat groups and the live feeds from videographers on the ground, these remote tacticians can direct the front-liners — and help guide their retreat.

As he spoke, the rest of the crew was entranced by a particularly graphic video game that Fat Boy was playing on a large-screen TV. His mother winced as a Wild West gunslinger lassoed a man and dragged him to his death.

“It’s so realistic,” she said.

Fat Boy seemed to appreciate the moment’s irony and briefly put down the controls.

“Now it’s like we are playing a video game on the street,” he said with a sigh. “Except it’s real, and it’s not fun.”

Image Demonstrators faced police officers in Kowloon in August.

Andrew Jacobs is a reporter with the Health and Science Desk, based in New York. He previously reported from Beijing and Brazil and had stints as a Metro reporter, Styles writer and National correspondent, covering the American South. @AndrewJacobsNYT

Tiffany May covers news in China and Asia, focusing recently on the protests in Hong Kong, where she is based. She graduated from Columbia University. @nytmay

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