Burning barricades at the entrance of a Hong Kong University during a protest

A Hong Kong police officer shot a masked protester in an incident shown live on Facebook Monday while a man was set alight, during one of the most violent days of clashes seen since pro-democracy unrest erupted more than five months ago.

Protesters, who had already begun a city-wide day of action aimed at paralysing the international financial hub, reacted to the morning shooting by rampaging through train stations, barricading streets and vandalising shops.

A masked assailant also doused a man with a flammable liquid and set him ablaze during an argument. The horrifying scene was captured on mobile phones and quickly went viral.

Police said a protester carried out the attack, and accused black-clad "rioters" of an array of other violent acts, including throwing a petrol bomb inside a train carriage.

Protesters voiced fury at what they said was excessive force used against them.

"Continuing this rampage is a lose-lose situation for Hong Kong," police spokesman John Tse said at a press conference in which he showed the video of the man being set alight, as well as a fire inside a train.

The footage of the shooting -- broadcast live to Facebook by a bystander -- showed a police officer drawing a pistol as he tried to detain a masked person at a junction that had been blocked by protesters.

Another unarmed masked individual then approached the officer and was shot, quickly falling to the ground.

Seconds later, two more live rounds were fired as the officer scuffled with another masked protester, who fell to the floor. Both were detained by officers.

A pool of blood could be seen near the first man, whose body initially appeared limp, although he was later filmed conscious and even trying to make a run for it.

The second man was conscious, shouting his name to reporters as he was handcuffed.

Hong Kong police confirmed one person was struck by a bullet, while hospital authorities said a 21-year-old man was admitted with a gunshot wound.

The man attacked by being set alight was taken to hospital in a critical condition, according to Tse.

Increasing violence

Hong Kong has endured 24 straight weeks of increasingly violent rallies aimed at securing greater democratic freedoms from China, which has ruled the city under a "one country, two systems" framework since its handover from the British in 1997.

The protesters are desperate to stop what they see as Beijing's tightening control over Hong Kong and its reneging on its handover commitment to allow greater liberties for the city than those on the mainland.

But Beijing has refused to give in, and instead warned it is prepared to impose even greater security measures.

Protesters have been incensed at what they see as police brutality and Monday's shooting only fuelled the tinderbox atmosphere.

"I don't understand why the police has to use that kind of brutality to hurt innocent people. I think it's just out of sense, out of control," a 22-year-old IT worker, who gave her surname as Chan, told AFP as she joined angry crowds in Sai Wan Ho after the shooting.

Monday's shooting was the third time protesters had been shot with live rounds by police. None of the shootings have resulted in deaths.

Tensions in Hong Kong were already high following the death on Friday of a 22-year-old student who succumbed to injuries sustained from a fall in the vicinity of a police clearance operation a week earlier.

After a weekend of clashes and huge vigils, Monday's chaos began with small groups of masked protesters hitting subway stations and roads during the rush hour commute.

But as footage of the shooting went viral, the protests snowballed.

During the lunchtime break in Central, a downtown district that hosts blue-chip international conglomerates and luxury retailers, police fired tear gas to disperse thousands of protesters, many dressed in office attire, chanting "murderers" and "Triads".

Tear gas and rubber bullets were fired in multiple districts throughout the morning, including at two university campuses.

One video circulated by protesters on messaging channels showed a police officer trying repeatedly to drive his motorbike into protesters who had gathered on a road.