HONG KONG — For months, clashes between police and pro-democracy protesters here proceeded in a worrisome but almost predictable rhythm: Water cannons and tear gas dispersed throngs of masked demonstrators only to find them amassing again the next weekend despite an intensifying crackdown by the pro-Beijing government.

That uneasy rhythm was jolted violently Monday.

In the morning, a police officer, claiming self-defense, shot a young, apparently unarmed protester in the abdomen at point-blank range, unleashing a chain of chaotic events as thousands of demonstrators clashed with riot police in the city’s financial district and violent confrontations erupted at university campuses. Hours later, a man who was castigating protesters purportedly involved in vandalizing a rail station was doused with a flammable liquid and set on fire. Authorities later identified the victim as a 57-year-old construction worker and said they are investigating the incident as an “attempted murder.” He was in critical condition.

The immolation prompted Hong Kong Chief Executive Carrie Lam to tag the protesters as “the people’s enemy” — words heard often from Beijing for those targeted for incarceration or worse. Lam, who said about 60 people were injured in Monday’s clashes, said protesters were “destroying society.” The government, she vowed, would not bow to such pressure.

The unrest, which continued Tuesday morning at several Hong Kong universities, marks the worst violence in the city in decades, posing a quandary for China’s leader, Xi Jinping, who has sought to bring Hong Kong to heel without resorting to Tiananmen Square-style bloodshed. That confrontation left hundreds dead when the Chinese army beat, shot and crushed people gathered to protest government oppression.

Few fear a repeat of Tiananmen Square in Hong Kong, in part because social media and the ability to send images at lightning speed to a watching world may serve as a deterrent against such extreme government violence. But the developments on Monday — captured on videos that quickly went viral — have led many to worry that the government in Hong Kong may now feel justified, if not emboldened, to use ever more severe methods to put an end to the protests, once and for all.

“Senior officials have issued very draconian comments regarding the promulgation of a national security law and stepping up overall control,” said Willy Wo-Lap Lam, a professor of Chinese politics at the Chinese University of Hong Kong. “This, together with the death of the student protester last week, is responsible for today’s outbreak of disorder.”

Student protesters “see no future ahead of them” because of the government’s crackdown and refusal to compromise, added Lam (no relation to Carrie Lam). “It seems like Beijing wants to use [the escalating protests] as an excuse to impose tougher measures.”

In Beijing, Foreign Ministry spokesman Geng Shuang declined to comment on the shooting of the protester, referring reporters to other government departments. The State Council office responsible for issues pertaining to Hong Kong did not respond to a request for comment.

“It’s a police state in Hong Kong,” said Jerry, 26, a finance worker who joined the protests and gave only one name out of fear of retribution. “Police are murderers.”

This is not the first time Hong Kong has experienced violence since the protests began this summer. A young woman became a symbol of the pro-democracy demonstrations after being shot in the eye by a rubber bullet fired by police. A protester was shot in a confrontation with an officer last month, and another fell to his death under questionable circumstances last week. Protesters have set fires in subway stations and vandalized businesses they suspect are owned by those sympathetic to Beijing.

Throughout more than five months of unrest, Beijing has exhorted Hong Kong’s leaders to clamp down harder on the dissenters. Hong Kong authorities have obliged with thousands of arrests, draconian new laws, a barrage of tear gas and the detention of pro-democracy lawmakers.

Yet far from blunting the democracy movement, the intensifying crackdown has prompted protesters to adopt more-aggressive tactics. With the deeply divided city descending into disorder, there has been no sign that Beijing might change tack or allow the Hong Kong government to offer a political compromise.

Protests began in June when the Hong Kong leader tried to push through a now-withdrawn extradition proposal that would have allowed criminal suspects to face trial in mainland China. But the movement has widened into an uprising against Beijing’s encroachment on Hong Kong’s autonomy, encompassing demands for full democracy and police accountability.

The unrest has pushed the city into recession. On Monday, numerous shops were closed, train lines were shut, and many workers were unable to reach their offices. Universities canceled classes. Police said a gasoline bomb was thrown into a subway car. A police officer who rode a motorcycle into a crowd of demonstrators was placed on leave pending an investigation.

In central Hong Kong, as police retreated in vans at one point in the afternoon, crowds on the footbridges above chanted, “Fight for freedom! Stand with Hong Kong!” Other onlookers shouted and threw debris at the vans.

Protesters occupied a main thoroughfare, erecting barricades and setting fires near high-end hotels. As protesters blocked a road tunnel, they clashed with onlookers and taxi drivers. Some travelers abandoned their cars and walked with their suitcases.

In an editorial published Monday night, the nationalist Global Times newspaper compared the Hong Kong protesters to the Islamic State.

“We sternly condemn the mobs for their barbarity of setting those ordinary citizens, those who disagreed with them, on fire. Their appalling behavior has become indistinguishable from that of IS members,” the paper’s Chinese-language edition said.

At a news conference Monday, police defended the officer’s decision earlier in the day to open fire on the apparently unarmed protester, saying the demonstrator had wanted to take the officer’s firearm.

“He was under threat by two people; if he lost his gun, he would be under severe threat. Hence, he decided to fire,” Kwok Pak-chung, regional commander of Hong Kong Island, told reporters.

The condition of the man, who was struck in the abdomen, was not life-threatening, Kwok said.

In the United States, Congress is considering a bill that would pave the way for sanctions against people who undermine Hong Kong’s autonomy. The bill, approved unanimously by the House of Representatives, would require the U.S. government to consider annually whether it should continue to treat Hong Kong as a trading entity separate from mainland China in response to political developments. However, the bill is stuck in the Senate, where Majority Leader Mitch McConnell (R-Ky.) so far has declined to bring it to a debate.

In a statement, the U.S. State Department condemned the violence in Hong Kong, urged the government to “build on its dialogue” with the public, and said the United States is watching the situation unfold “with grave concern.” Department spokesperson Morgan Ortagus also urged Beijing to “honor the commitments” made to Hong Kong, including a high degree of autonomy, rule of law and civil liberties, which are “key to preserving its special status under U.S. law.”

Hong Kong is governed by a “one country, two systems” arrangement, under which Beijing pledged to maintain the territory’s relative freedoms and autonomy for half a century after its return to Chinese rule in 1997. But China has been tightening its grip, triggering anger in Hong Kong and uncertainty about its status as a global financial center.

Amnesty International branded Monday’s shooting as “another shocking low for the Hong Kong police” and called for an urgent independent examination.

In the meantime, protesters continue to turn their fury on police.

“They’re crazy. It’s outrageous,” said Kong, a 27-year-old woman on her lunch break, referring to Monday’s shooting. “They’ve lost control.”

David Crawshaw in Hong Kong and Liu Yang in Beijing contributed to this report.