HONG KONG — In a fearsome escalation of violence, Hong Kong police shot a protester at close range in the chest Tuesday, leaving the teenager bleeding and howling on the ground. Tens of thousands joined anti-government demonstrations that spread across the semi-autonomous Chinese territory even as Communist leaders in Beijing celebrated 70 years in power.

The single pistol shot fired by the officer as protesters swarmed toward him hit the 18-year-old on the left side of his chest, police spokeswoman Yolanda Yu said. She described the protesters as “rioters” and said the officer had feared for his life.

Hong Kong’s hospital authority said the teen was one of two people in critical condition, with a total of 51 people injured as fierce clashes between protesters and police wracked China’s freest and most international city.

While officers have previously fired warning shots in the air on multiple occasions during months of demonstrations in Hong Kong, this was the first time a protester is known to have been shot. There were other instances Tuesday when officers also drew their weapons, including two with bloodied faces who pointed pistols, as protesters determined to spoil the Oct. 1 anniversary of Communist rule fought pitched battles with riot police.

Video that spread quickly on social media appeared to show the officer opening fire as the protester came at him with a metal rod, striking the officer’s shooting arm.

Taken by the City University Student Union, it showed a dozen black-clad protesters hurling objects at a group of police and closing in on the lone officer who pointed his gun and opened fire. The protester toppled backward onto the street, bleeding from below his left shoulder.

As another protester rushed in to try to drag away the wounded youth and was tackled by an officer, a gasoline bomb landed in the middle of the group of officers in an explosion of flames.

The shooting marked a dramatic escalation in violence that spread chaos to multiple areas. Widespread fighting and destruction prompted an ominous warning Tuesday evening from Hong Kong’s embattled police force, now widely decried for heavy-handed tactics, that rioters posed “a serious threat to public peace and order.” Protesters used power tools to fashion bricks into missiles and came armed with gas bombs.

“Whilst there is no excuse for violence, the use of live ammunition is disproportionate, and only risks inflaming the situation,” U.K. Foreign Secretary Dominic Raab said of the protests in the former British colony that returned to Chinese rule in 1997.

Riot police fired tear gas in at least six locations and used water cannons in the business district as usually bustling streets became battlefields. Hundreds of protesters sprinted into a Methodist church for refuge as officers firing volleys of choking gas swept through streets near police headquarters. A clergyman at the door clutched a bottle of water to wash out stinging eyes.

Determined to thumb their noses at Chinese President Xi Jinping, protesters ignored a security clampdown that saw more than two dozen subway stations closed, with some trekking for miles (kilometers) in the tropical heat and humidity to join a massive march in the city center.

Chanting anti-China slogans and “Freedom for Hong Kong,” the dense crowd dressed in mournful black snaked for over a mile (1.6 kilometers) along a broad thoroughfare in defiance of a police ban. Some carried Chinese flags defaced with a black cross. Organizers said at least 100,000 people marched. Police didn’t give an estimate.

“They are squeezing our necks so we don’t breathe the air of freedom,” said King Chan, a 57-year-old homemaker who marched with her husband.

Demonstrators tossed wads of fake bank notes usually used at funerals into the air. “The leaders who won’t listen to our voice, this is for them,” said marcher Ray Luk.

Thousands confronted police across the city, the largest number of simultaneous protests since the unrest began in early June over a now-shelved extradition bill that activists say was an example of how Hong Kong’s freedoms and citizen rights are being eroded. The movement has since grown into an anti-Chinese campaign with demands for direct elections for the city’s leaders and police accountability.

The smell of stinging tear gas and smoke from street fires started by protesters engulfed the Wan Chai, Wong Tai Sin, Sha Tin, Tuen Mun, Tsuen Wan and Tsim Sha Tsui areas.

Protesters used umbrellas as shields and threw tear gas canisters back at police. Police said protesters used corrosive fluid in Tuen Mun, injuring officers and some reporters.

In Wong Tai Sin, a gasoline bomb that protesters hurled at police exploded near motorcycles parked along a pavement, creating a large blaze later extinguished by firefighters. Some protesters placed an emergency water hose down a subway station to try to flood it.

A water cannon truck sprayed blue water, used to identify protesters, to disperse crowds from advancing to government offices. Scores of police also stood guard near Beijing’s liaison office as the battles continued.

“Today we are out to tell the Communist Party that Hong Kong people have nothing to celebrate,” said activist Lee Cheuk-yan as he led the downtown march. “We are mourning that in 70 years of Communist Party rule, the democratic rights of people in Hong Kong and China are being denied. We will continue to fight.”

Activists carried banners saying, “End dictatorial rule, return power to the people.”

Distributing funeral bank notes to marchers, black-clad protester Julie Milde said she was “mourning for our Hong Kong. It’s going to be the death soon.”

The popular LIHKG online chat forum used by protesters was inaccessible on cellphones, a move believed to have been aimed at thwarting their communications.

As protesters hoped, the chaos in Hong Kong contrasted with anniversary festivities in Beijing, which included a muscular parade of military might. Among those attending was Hong Kong Chief Executive Carrie Lam, whose leadership during the crisis has made her a hate-figure for many protesters.

As the city’s government marked the anniversary with a solemn ceremony in the morning, police used pepper spray to break up a brief scuffle between Beijing supporters and a small group of pro-democracy protesters.

Hong Kong Chief Secretary Matthew Cheung told hundreds of guests at a reception that the city has become “unrecognizable” due to the violence.

Cheung said Beijing fully supports the “one country, two systems” framework that gives Hong Kong freedoms and rights not enjoyed on the mainland.

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