Violence coursed through Hong Kong on National Day with dozens hurt and more than 150 people arrested. (AFP)

Hundreds of Hong Kong citizens staged a sit-in on Wednesday outside the school of a protester who was shot by police as authorities said the wounded 18-year-old was now in a stable condition.

The international finance hub has been left reeling from the shooting, the first time a demonstrator has been struck with a live round in nearly four months of increasingly violent pro-democracy protests.

Hong Kong was battered by the most sustained political clashes of the year on Tuesday as China celebrated 70 years of Communist Party rule with a massive military parade in Beijing.

The spiraling violence underscored seething public anger against Beijing’s rule and shifted the spotlight from China’s carefully choreographed birthday party, which was designed to showcase its status as a global superpower.

Running battles raged for hours across multiple locations as hardcore protesters hurled rocks and petrol bombs. Police responded for the most part with tear gas, rubber bullets and water cannon.

In Tsuen Wan district, a police officer fired his weapon at close range into the chest of Tsang Chi-kin, 18, as his unit was attacked by protesters armed with poles and umbrellas.

Police said the officer feared for his life on a day that saw his colleagues fire five warning shots from their pistols throughout the city.

“In this very short span of time, he made a decision and shot the assailant,” police chief Stephen Lo said.

But protest groups said the officer charged into the melee with his firearm drawn and condemned the increasing use of live rounds.

“HK (has) fallen into a de facto police state,” democracy activist Joshua Wong tweeted. “The paramilitary security forces completely took over this city.”

The shooting was captured on video that quickly went viral.

Outside Tsang’s school on Monday students chanted slogans and held pictures of the incident, taken form videos posted on Facebook.

“No rioters, only tyranny,” they chanted, alongside other popular protest slogans.

Tsang, who was filmed trying to strike the officer with a pole as he was shot, was taken to a nearby hospital in a critical condition but authorities said his condition has since improved.

“According to the latest information of the Hospital Authority, the current condition of the man is stable,” the government said in a statement.

A friend and classmate of Tsang, who gave his first name Marco, said the 18-year-old was a keen basketball player who was infuriated by sliding freedoms in Hong Kong and the police response to the protests.

“If he sees any problems or anything unjust, he would face it bravely, speak up against it, instead of bearing it silently,” Marco told AFP.

Police said 25 officers were injured in the National Day clashes, including some who suffered chemical burns from a corrosive liquid that was thrown at them by protesters. The liquid also wounded some journalists.

Hospital authorities said more than 70 people were admitted on Tuesday. Police made some 160 arrests throughout the day.

Hong Kong’s protests were ignited by a now-scrapped plan to allow extraditions to the mainland.

But after Beijing and local leaders took a hardline they snowballed into a wider movement calling for democratic freedoms and police accountability.

With Hong Kong leader Carrie Lam seemingly unwilling or unable to find a political solution, police have been left to battle increasingly radicalized protesters.

Protesters and some local residents routinely shout “triads” at officers often respond by calling demonstrators “cockroaches”.

The protest movement’s main demands are an independent inquiry into police actions, an amnesty for those arrested and universal suffrage.

But Beijing and Lam have said they are unwilling to meet those demands.