China warned on Monday that it will not sit back and watch Hong Kong's protests develop into an "uncontrollable" situation as dozens of protesters occupying a university made a wavy escape from the police.
Hong Kong police fired tear gas and water cannon at protesters who tried to break through the police line and reach the Hong Kong Polytechnic University to reinforce protesters in the middle of a week-long balancing act with security forces.
Black-clad protesters threw gas bombs as they tried to come to school in the most intense violence during five months of anti-government demonstrations.
The South China Morning Post reported that a large fire was burning inside the campus and loud explosions were heard late Monday.
Dozens of protesters escaped the besieged university by descending on a rope from a footbridge to a highway. Once on the road, they were seen to be picked up by waiting motorcyclists and flee.
Monday's violence drew strong punishment from China's ambassador to Britain, Liu Xiaoming.
"I think the Hong Kong government is trying very hard to put the situation under control," Liu told a press conference in London. "But if the situation becomes uncontrollable, the central government would certainly not sit on our hands and watch. We have enough resolution and power to end the unrest. "
The comments came after a brief and rare deployment of Chinese troops in Hong Kong over the weekend to clean up the streets. The People's Liberation Army has barracks in an upper-class district in Hong Kong. [1
Liu also warned of "external interference" in internal Chinese affairs and pronounced Britain and the United States at the same time as they accused them of condoning violence by protesters.
Al Jazeera & # 39; s Rob McBride, reporting from outside Hong Kong University, said there were a few thousand people who arrived and a fight between security forces and those who tried to enter the school soon followed.
"There has just been a terrible collision between protesters who ade gathered near the campus to try to support the besieged students. The water cannon's violence and tear gas I think surprised many people, says McBride.
Hong Kong police accused former protesters of using dangerous tactics ics against the insurgency police.
"The most alarming deadly weapons were used at Polytechnic University, which has become a weapons factory and a refuge for extremely violent insurgency," said Kwok Ka-Chuen, the police spokesman.
"Violence has reached a critical level. Insurgency is intended to murder our officers at all costs."
Police have been attacked with bows and arrows, Molotov cocktails and catapults shooting bricks and other debris in recent days.
Lui accused protesters of trying to destabilize and paralyze Hong Kong to "seize power" in a drive for independence.
"Hong Kong citizens live under black terror created by violent extremism with their lives and property under severe threat," he said.
Hong Kong's economy has plunged into a technical recession because of the protests over the months, killing its international image as a place to do business, Lui said.
"The future of Hong Kong, if such a situation continues, would be unimaginably terrible."
To the "rescue"
Earlier in the day, hundreds of protesters made their way to the university, trying to break through police protection. Protesters blocked a highway not far from campus to distract police and help those inside the campus escape.
"We've been trying to save them all day," said a young man in a blue T-shirt, hat and glasses, driving down Nathan Road, the Kowloon district's largest commercial street. "They're trapped in there."
The size of the demonstrations has decreased in recent weeks, but collisions between protesters and police have escalated sharply since the beginning of last week, when police fired a protest, a man was set on fire.  On Monday night, protesters hugged under umbrellas along the median strip of Nathan Road and filled bottles of gasoline to make raw bombs, a weapon they have been increasingly using.
Some residents were caught by police protection and all shops along a stretch of commercial strip which is usually one of Hong Kong's busiest were closed.
Police say 4,491 people aged 11 to 83 have been arrested since protests began in June.
Protesters are angry at what they see as Chinese blend in Hong Kong's promised freedoms when the then British colony returned to Chinese rule in 1997. They say they respond to excessive use of force by the police.
China states that it is engaged in "one country, two systems" formula that grants Hong Kong autonomy. City police have denied accusations of brutality, saying they are showing restraint.