Hong Kong, Nov 12 (AFP) Hong Kong protesters struck the city's transport network for a second day running on Tuesday as western powers voiced concern over spiralling violence after police shot a young demonstrator and another man was set on fire.

Small bands of masked protesters blocked roads, threw objects onto train tracks and held up subway trains, sparking cat and mouse clashes with riot police and renewed chaos on the morning commute. Universities were also a flashpoint with police firing tear gas around at protesters who had blocked roads leading to the Chinese University of Hong Kong.

Masked activists outside Hong Kong University blocked a main road by dropping objects from a footbridge. Overnight, students there had clashed with police after officers made an arrest outside one of the university's residences.

And at Polytechnic University clashes broke out as police tried to arrest a female student.

Owan Li, the student representative on the university's governing body, said he was beaten by police as he tried to mediate. "The officers were rather arrogant and unbridled," Li told AFP, sending pictures of bruises to his hand and leg.

"They said I was being aggressive and obstructing their work, then the six to seven of them dragged me out and gave me a round of beating and kicking."

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 reneging on its handover commitment to allow greater liberties for the city than those on the mainland.

On Monday the financial hub was convulsed by some of the worst violence yet as crowds reacted to the shooting of a 21-year-old protester by rampaging through train stations, barricading streets and vandalising shops throughout the day and night.

Footage of the shooting -- broadcast live on 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 protester then approached the officer and was shot, quickly falling to the ground.

The video quickly went viral and inflamed already sky-high anger towards the police.

Protests raged for hours in multiple neighbourhoods, including a lunchtime rally mostly made up of office workers in the city's main commercial district, which was broken up by tear gas.

Horrifying footage also emerged of a man being doused with a flammable liquid and set ablaze by a masked assailant following an argument with pro-democracy protesters.

Both the man set alight and the shot protester remained in critical condition on Tuesday, hospital authorities said.

The violence prompted western powers to urge Beijing and Hong Kong leader Carrie Lam to find a compromise with protesters who are seeking greater democratic freedoms and police accountability.

"We condemn violence on all sides, extend our sympathies to victims of violence regardless of their political inclinations, and call for all parties -- police and protesters -- to exercise restraint," State Department spokeswoman Morgan Ortagus said in a statement. Britain said the latest violence was "deeply disturbing".

"Political dialogue is the only way forward and we want to see the Hong Kong authorities agree a path to resolve this situation," Downing Street said in a statement.

But there is little sign Beijing or Lam are willing to offer any political concessions.

As clashes raged on Monday, Lam said protesters were indulging in "wishful thinking" if they thought violence would achieve political change.

On Tuesday morning she gave another press conference criticising protesters and praising people who tried to make it into work. (AFP)

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