Hong Kong pro-democracy protesters defied a warning by China's President Xi Jinping and took to the streets again Friday, as the political turmoil seeped out to London where a minister from the territory was confronted by masked demonstrators.

Hong Kong: Hong Kong pro-democracy protesters defied a warning by China's President Xi Jinping and took to the streets again Friday, as the political turmoil seeped out to London where a minister from the territory was confronted by masked demonstrators.

Hong Kong has seen relentless protests since June as many in the city of 7.5 million people have vented fury at eroding freedoms under Chinese rule. Violence has escalated, and tensions have spilt out overseas, sparking friction between China and Britain, which governed Hong Kong until 1997.

On Thursday, Hong Kong justice secretary Teresa Cheng fell in London after being surrounded by pro-democracy protesters, in the most physical confrontation involving a member of cabinet since the unrest began.

Cheng walked away without any visible signs of injury. But China called it an "appalling attack" and accused Britain of fuelling the protest movement. Police in London said they were investigating the incident.

Former colonial power Britain has urged Beijing and Hong Kong to seek a political solution to the city's crisis and has condemned the escalating violence on both sides. In a briefing in Beijing, Chinese foreign ministry spokesman Geng Shuang said if the UK "continues to add fuel to the fire... then it will bring calamity on itself."

Earlier on Friday, thousands of mainly office workers took to the Chinese territory's streets, many chanting "Stand with Hong Kong" and raising an open hand with five fingers splayed. It is a reference to the five demands of the protest movement, which include the right to freely elect Hong Kong's leaders, as well as an independent inquiry into alleged police brutality.

"Every person in Hong Kong has a role to play," James, a 33-year-old banking employee told AFP, adding "sacrifice" was necessary to keep the wind behind the protest movement.

Black-clad protesters also occupied university campuses, while the city endured another day of transport chaos with suspensions on the vandalised train network and roads blocked by barricades.

Firefighters battled to quell flames from a car set ablaze near the Chinese University of Hong Kong late Friday.

Their actions were in defiance of a warning by Chinese President Xi Jinping, who on Thursday backed Hong Kong's leader Carrie Lam and the police force, while warning the protest movement was threatening the "one country, two systems" principle governing the semi-autonomous city.

Xi said "stopping violence and controlling chaos" was the top priority.

With the crisis deepening by the week, fears have grown that Xi's patience will run out and Chinese troops will be deployed to Hong Kong.

The Global Times, one of the powerful arms of the Chinese state media, on Thursday fuelled tensions with a tweet reporting that a curfew was imminent.

But it quickly withdrew the tweet and Hong Kong's government denied a curfew was planned.

With China offering no concessions, protesters switched tactics on Monday when they launched a "blossom everywhere" campaign to cause as much disruption as possible and overwhelm the police force.

Until this week, protests had been mainly in the evenings and on weekends, allowing the international financial hub to still function relatively smoothly during the week albeit with its economy dragged into recession.

But roadblocks and vandalism to metro stations and lines have brought chaos to the city transport network, forcing schools to close and many commuters to stay at home.

Organisers of Clockenflap, Hong Kong's biggest music festival, said Friday that this year's edition has been cancelled because of the unrest.

Protesters' actions have been accompanied by intensifying violence from both sides -- two people have died in a week in incidents linked to the protests.

A 70-year-old man died on Thursday from injuries sustained a day earlier when he was hit by a brick during clashes between protesters and people angry with them.

Police on Friday said they had opened a murder case into the man's death.

Major universities have also become a hub for the protesters — the first time a movement characterised by its fluidity and unpredictability has coagulated in fixed locations.

A police spokesman described the occupation by black-clad protesters of the Chinese University of Hong Kong as "a powder keg".

But as dusk fell on Friday, the numbers of university students and protesters appeared to thin out.

"If just a small number of people stay here, it's easy for police to break in and arrest them," a student giving her name only as Wendy said.

The unrest was triggered by opposition to a Hong Kong government plan to introduce a law allowing the extradition of criminal suspects to mainland China.

The government belatedly withdrew the bill months into the unrest, but by then it had morphed into a much wider campaign for democratic freedoms and against the police.