Flash-mob rallies broke out in Hong Kong on Wednesday as anger mounted over police shooting a teenage protester who attacked officers in a dramatic escalation of the violent unrest that has engulfed the territory for months.

A large crowd of demonstrators, including office workers in shirts and suits, mustered in a park and then began marching through the city's commercial district in an unsanctioned rally, chanting anti-police and anti-government slogans.

Hundreds of students also staged a sit-in at the school of 18-year-old Tsang Chi-kin, who was shot in the chest by a policeman as he and a group of masked protesters attacked officers with umbrellas and poles.

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.

Tuesday witnessed the worst clashes of the summer as China celebrated 70 years of Communist Party rule with a massive military parade in Beijing.

The spiralling violence underscored seething public anger against Beijing's rule and shifted the spotlight from China's carefully choreographed birthday party.

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.

As criticism rose the police launched a spirited defence of their colleague at a press conference on Wednesday saying he feared for his life and the safety of his colleagues “He only had one option, that is to fire the gun to immediately resolve the danger,” deputy commissioner Mr Tang Ping-keung told reporters.

But protest groups said the officer charged into the melee with his firearm drawn and failed to fire a warning shot as they condemned the increasing use of live rounds.

“The people of Hong Kong are sick and tired of having mere words of condemnation as their only shields against lethal bullets and rifles,” a masked protester said at a press conference near Tsang's school.

Record arrests and tear gas

The shooting was captured on video that quickly went viral.

Opinion towards the shooting has largely cemented along ideological divides with pro-democracy activists condemning the police and establishment figures calling it a justified use of force.

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 had since improved.

A friend and classmate of Tsang, who gave his first name Marco, said the 18-year-old was a keen basketballer 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 30 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.

Outlining the sheer extent of Tuesday's clashes, police revealed that they arrested 269 people, ranging from 12 to 71 years old — the largest daily toll since protests began.

They also unleashed a record 1,400 rounds of tear gas, 900 rubber bullets, 230 sponge rounds, 190 bean bag rounds and five live rounds as warning shots.

To compare, police fired 1,000 tear gas canisters in the first two months of the protests.

Hong Kong's courts are rapidly filling up.

On Wednesday, 96 protesters arrested on Sunday appeared in court charged with rioting.

Ages ranged from 14 to 39 and while the majority were students in their twenties, other occupations listed included a waitress, a teacher, a doctor, an advertising executive and a cook.

The protests were ignited by a now-scrapped plan to allow extraditions to the mainland.

But after Beijing and local leaders took a hard line 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 radicalised protesters.

Sentiment is hardening on all sides.

Protesters and some local residents routinely shout “triads” at officers who 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.