Hong Kong, Nov 12 (AFP) Hong Kong pro-democracy protesters fought intense battles with riot police on a university campus and paralysed the city's upmarket business district Tuesday, extending one of the most violent stretches of unrest seen in more than five months of political chaos.

The confrontations followed a particularly brutal day on Monday, when police shot a protester and a man was set on fire, prompting calls from western powers for compromise but further fury in China against any challenge to its rule.

The city's universities emerged as a new flashpoint throughout Tuesday with sustained clashes at major campuses for the first time.

The epicentre was the Chinese University of Hong Kong where the usually placid hillside grounds were turned into a battlefield.

Police fired repeated volleys of tear gas and rubber bullets at hundreds of protesters who had built barricades ending an hours-long stand-off between the two sides.

Protesters responded with bricks and petrol bombs, while a vehicle used in a barricade was set alight.

After a brief hiatus in which faculty staff tried to mediate, new clashes broke in the evening with flames lighting up the night sky and fresh rounds of tear gas whizzing through the air.

Police used a water cannon truck in an attempt to dislodge protesters but they remained behind their makeshift shield walls.

There were smaller clashes on at least three other university campuses throughout the day.

After months of largely confining their most disruptive protests to the weekends, online groups used by protesters have been pushing new tactics targeting the working week.

For the last two days flashmob protesters have blocked roads during the morning rush hour, thrown objects onto rail tracks and held up subway trains, sparking transport chaos throughout the city.

In Central, a district that is home to many blue-chip international firms and luxury shops, thousands of office workers occupied roads for hours on Tuesday chanting: "Fight for freedom, stand with Hong Kong!".

Hundreds of hardcore protesters threw bricks and other objects before retreating when riot police fired tear gas in the shadows of high-end stores.

But the streets filled with protesters once more as people left work for the evening, prompting police to carry out another dispersal operation with tear gas.

The scenes in Central were a vivid illustration of how ordinary people are continuing to back the pro-democracy movement -- even as the radicals adopt more violent tactics.

Authorities have offered repeated condemnation but been unable or unwilling to find a solution to the crisis.

"Hong Kong's rule of law has been pushed to the brink of total collapse," police spokesman Kong Wing-cheung told a press conference on Tuesday as he defended the force against seething public anger.

China has ruled Hong Kong under a "one country, two systems" framework, which allows the city greater freedoms than on the mainland, since its handover from the British in 1997.

The protest movement has been fuelled by Beijing's tightening control over Hong Kong.

Protesters are demanding a right to freely elect their leaders, as well as an independent inquiry in what they see as police brutality.

But China has steadfastly refused to give any concessions to the protesters, and instead warned of even tougher security measures.

Beijing has repeatedly signalled that it would be prepared to send mainland security forces into Hong Kong.

Chinese state media on Tuesday again raised the spectre of the People's Liberation Army being deployed to end the crisis.

The warnings were in response to Monday's violence, in which a man was doused with a flammable liquid and set ablaze by a masked assailant following an argument with pro-democracy protesters.

Chinese authorities, as well as state-run media, have said the attack was an example of protesters' violent tactics, although the assailant has not been arrested and his identity remains unknown.

"This kind of hair-raising behaviour has caused terror and anxiety among the broader Hong Kong public," foreign ministry spokesman Geng Shuang told reporters in Beijing.

"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," US State Department spokeswoman Morgan Ortagus said in a statement.

They were reacting in fury to a police officer shooting a 21-year-old protester on Monday morning, with that incident broadcast live on Facebook by a bystander.

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

Tensions had initially spiked following the death last week of a young man who fell from a multi-storey car park during clashes with police.

He was the first student to die in the five months of protests, and protests over the weekend then escalated at the start of the working week.(AFP)

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