HONG KONG — In a dramatic escalation of violence, Hong Kong police shot a protester in the chest at close range 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 in a challenge to Beijing’s dominance as the Communist Party celebrated 70 years in power.

The officer fired the single pistol shot as protesters swarmed toward him, Police Commissioner Stephen Lo said, hitting the 18-year-old on the left side of his chest. Lo defended the action as “reasonable and lawful,” saying the officer feared for his life.

Hong Kong’s hospital authority said the teen was one of two people in critical condition, with 66 injured as clashes between protesters and police wracked China’s freest and most international city.

While officers have fired warning shots in the air on multiple occasions during months of unrest, this was the first time a protester has been struck by gunfire. The shooting marked a dramatic surge in violence that spread multiple areas.

Lo said there was no order for police to shoot if they are under threat, but they can use appropriate force. He described protesters as “rioters,” saying they have committed widespread criminal acts — from attacking police officers, including 25 who were injured, to destroying public property and vandalizing shops and banks linked to China.

“The officer was under attack, his life was threatened. He made a very quick decision and shot the assailant. I believe it was his best judgment at the time,” Lo said.

He added that although the officer also had a rifle for rubber bullets, the event unfolded quickly.

Apart from the incident in Tsuen Wan, where the teen was shot, he said officers also fired five warning shots in four other areas, although no one was injured. Police arrested more than 180 protesters, he added.

“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.

As protesters hoped, the chaos in Hong Kong contrasted with anniversary festivities in Beijing, which included a muscular parade of military might.

In Beijing, President Xi Jinping stood in the very spot overlooking Tiananmen Square where Mao Zedong had declared the establishment of the People’s Republic in 1949, following in the footsteps of the Soviet Union.

Meanwhile, President Donald Trump congratulated China on the 70th anniversary, a message posted hours after the protester in Hong Kong was shot.

Trump’s cheerful commemoration in a one-sentence tweet to “President Xi and the Chinese people” stood in contrast to the condemnation aimed at Chinese leadership from fellow Republicans, and it drew condemnation from human rights groups.

The Washington Post contributed.