Hong Kong police shot a teenage protester in the chest from close range on Tuesday, as violence rocked districts all across the city on the day China celebrated 70 years of Communist Party rule.

The shooting marked a dramatic escalation in the four months of increasingly violent protests, as demonstrators ignored police warnings and thronged the streets to join “national grief” rallies to mourn their lack of democracy and vent their anger at Beijing’s rule.

As Chinese president Xi Jinping took salutes from goose-stepping troops at the celebratory parade in Beijing, protesters were hurling eggs at his portrait in Hong Kong.

Pro-democracy lawmakers in Hong Kong claimed police have “gone out of control” following the shooting, as the pro-Beijing camp demanded the introduction of stringent emergency laws to finally quell the unrest that has convulsed the Asian financial hub for four months.

Protesters ran simultaneous marches and rallies across the city in an effort to stretch police resources, and by early afternoon there were street fires raging in at least a dozen districts.

The city was a battlefield as protesters lit bonfires, barricaded roads, and attacked police with bottles, bricks, poles and petrol bombs. The riot police came out in huge numbers to combat them, and swept through the city firing water cannons, tear gas and plastic bullets.

It was in one of these clashes that an 18-year-old secondary school student was shot in the chest.

As the bonfires smouldered into the night, the protesters tallied up their losses for the day: one shot; another 50 hospitalised; more than 180 arrested.

Outside one police station a small group continued to lob bricks over the high fence. The tear gas warning sign was raised from behind the gates and one protester laughed. He couldn’t count how many times he had seen that sign through the day.

What did today mean to him?

“War,” he said. “It just means war.”

Captured on video

The earlier shooting was captured on video by a student: it shows a group of activists chasing a policeman in an area where police and protesters had been engaged in running battles for some hours.

The policeman fell to the ground and the protesters started beating him with poles, as other police rushed to his assistance. One protester went to strike another policeman on the arm with a pole, and the policeman raised his gun and shot him from very close range.

The young man stumbled backwards and fell over the officer on the ground. As the other protesters withdrew, a petrol bomb was thrown and erupted into flames at the feet of the regrouping police.

The injured man was seen groaning on the ground, repeatedly saying: “My chest is in pain. Take me to the hospital.”

He underwent surgery on Tuesday night and is reported to be in a critical condition.

A hospital spokesman said the bullet had missed the man’s heart and was lodged in his left lung. “If the operation goes smoothly, given his age and general condition, there is a good chance he will survive,” the spokesman said.

A police spokeswoman said on Tuesday night that the force was “saddened” the man had been shot, but “as an officer felt his life was under serious threat, he fired a round at the assailant to save his own life and his colleagues’ lives”. She added that 25 officers had been injured in the course of the day.

While the police have fired several warning shots during these clashes over the past four months, this is the first time someone has been hit with a live round.

On the other side of Victoria Harbour, the day’s biggest rally fell eerily quiet as word of the shooting filtered through the crowd. The protesters absorbed the news as phalanxes of police approached.

“We knew it would come to this, but it is still such a shock,” one young woman said. “It’s like we know some of us will get arrested, put in jail. But when friends are actually taken away, I can’t really believe it.”

A group of 24 pro-democracy lawmakers issued a joint statement condemning the police, saying “The policeman’s close-range shooting seems to be an attack rather than self-defence . . . Many police officers have gone out of control.”

Police detain an anti-government protester in Hong Kong on Tuesday. Photograph: Felipe Dana/AP

They also criticised chief executive Carrie Lam for spending the National Day in Beijing at the parade in Tiananmen Square, even though the Hong Kong police had warned in advance they expected chaos.

They called her absence “tantamount to authorising police to administer Hong Kong”.

Amnesty International Hong Kong said on Tuesday night that the shooting “marks an alarming development” and “police should only use lethal force in response to an imminent threat of death or serious injury and only as a last resort.”

Defending the police

Meanwhile, several pro-Beijing legislators defended the police on Tuesday and demanded Mrs Lam’s government invoke colonial-era emergency regulations laws which would give the government wide-ranging powers to crack down on suspects.

Police detain a protester on a road in the Wanchai area of Hong Kong. Photograph: Nicolas Asfouri/AFP/Getty Images

Among the lawmakers calling for the introduction of new laws was Junius Ho, who is reviled by the protesters as they allege he arranged for triads to attack them during rallies. His office was ransacked during the protests on Tuesday, for the third time since the demonstrations began.

“There is no sign of this coming to an end until the government is determined to clamp down,” he said, “the government must call for emergency measures and restore law and order.”

Many human rights groups are strongly opposed to the introduction of the emergency laws, which date back to 1922, as they say they are outdated, draconian and susceptible to abuse.

The protests started in early June as a demonstration of anger against a proposed piece of legislation that could have seen suspects extradited to China to face the Communist Party’s opaque legal system.

While that law has now been scrapped, the protests became a wider issue with demands quickly including an investigation into police brutality, an amnesty for all arrested protesters and the introduction of truly democratic elections.

The protests show no sign of abating and the shooting is likely to add fuel to the fire.