Dozens of Hong Kong protesters have staged a dramatic escape from a besieged university campus, shimmying down plastic hosing from a bridge and fleeing on waiting motorbikes as the police fired projectiles.

Key points: Hong Kong's leader urged police to act humanely as they pressed their siege of Polytechnic University

Hong Kong's leader urged police to act humanely as they pressed their siege of Polytechnic University Protesters escaped on Monday night by lowering themselves on hoses and fleeing on waiting motorbikes

Protesters escaped on Monday night by lowering themselves on hoses and fleeing on waiting motorbikes Witnesses say there are still around 300 people, including children, trapped on the campus

However, many protesters remained trapped inside Hong Kong Polytechnic University, and police let two prominent figures onto the campus late on Monday to mediate in an attempt to break the deadlock.

Today the city's embattled leader Carrie Lam said about 100 protesters remained inside the campus. She said 600 people had left, including 200 who were under 18 years old.

Ms Lam said those under 18 would not be immediately arrested, but could face charges later.

Hong Kong Polytechnic University is located in Hung Hong, in Hong Kong. ( ABC News )

She said the violence afflicting the former British colony exceeded protesters' demands for democracy, and described demonstrators as now being the "people's enemy".

But she also said police should act humanely in bringing the campus siege to an end.

Police surrounded the university on Sunday and have been laying siege to it ever since, using tear gas and rubber bullets while themselves coming under fire from protesters armed with bricks, petrol bombs and even bows and arrows.

Many protesters tried to escape from a sewage tunnel inside the Hong Kong Polytechnic University campus. ( Reuters: Adnan Abidi )

Amid the standoff, China's State Council appointed Tang Ping-keung, also known as Chris Tang, as Hong Kong's new Police Chief.

Ahead of Mr Tang's swearing in, which follows the retirement of Stephen Lo from the position on Monday, he told the South China Morning Post the 31,000-strong force could not end the city's unprecedented social unrest on its own.

He also warned "fake news" was undermining the reputation of his police force.

Hong Kong Polytechnic University has been at the centre of the standoff in the past week and has seen the most intense violence in five months of anti-government demonstrations.

Some of the protesters who escaped on Monday night did so by lowering themselves about 10 metres from a bridge they had occupied on the campus to a flyover below.

People waiting on motorbikes on a nearby flyover facilitated some escapes. ( AFP: Ye Aung Thu )

They then sped off on the back of motorcycles which were already waiting or quickly arrived.

A number of them appeared subsequently to have been arrested, a witness told Reuters.

Other protesters, hurling petrol bombs, tried repeatedly to break into the campus, but police fired tear gas and a water cannon to push them back.

The size of demonstrations has dwindled in recent weeks, but clashes between protesters and police have worsened since early last week, when police shot a protester, a man was set on fire and the city's financial district was filled with tear gas in the middle of the workday.

The city's hospital authority reported 116 injuries on Monday, including one female in a serious condition.

Demonstrators set Hong Kong Polytechnic University's main entrance ablaze to prevent surrounding police moving in. ( AFP: Ye Aung Thu )

Cordon tightened in uni siege, 'children trapped' inside

Police detained protesters after they tried to escape the campus of the university the Hung Hom district of Hong Kong. ( AFP: Anthony Wallace )

A day after the High Court ruled a ban on facemask was unconstitutional, China's top legislature said Hong Kong courts had no power to rule on the constitutionality of the city's legislation, according to state media outlet Xinhua.

Earlier on Monday police had tightened their cordon around the Polytechnic University, and fired rubber bullets and tear gas to pin back a few hundred anti-government protesters armed with petrol bombs and other weapons.

Dozens, choking on the tear gas, tried to leave the campus by breaking through police lines, but were pushed back.

"If the police decide to come in by force, to make their arrests then there will be very strong resistance from the protesters, and we are afraid we may see bloodshed. This is something that we want to avoid," Mr Tsang said.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 17 seconds 1 m 17 s Violent scenes outside Hong Kong Polytechnic University.

"The situation is getting more and more dangerous," said Jasper Tsang, a pro-Beijing politician who is the former head of Hong Kong's Legislative Council, speaking to Reuters soon after he arrived at the campus.

As he spoke, big explosions were heard and flames flared up at a distant part of the campus.

Mr Tsang, who with legal scholar Eric Cheung was the first prominent mediator allowed by police to enter the campus, said there were young children and elderly people trapped inside, and that it was a priority to get the children out first.

Police said officers had been deployed "on the periphery" of the campus for a week, and appealed to "rioters" to leave.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 43 seconds 43 s Hong Kong police respond to university clashes

"All roads to Poly U are blocked," said a policeman who stopped Reuters reporters at a road block on Monday night. "All are blocked."

On Monday night witnesses had estimated there were more than 300 people still on the campus.

Police warned they were ready to use live bullets if protesters continued to use lethal weapons. ( Reuters: Tyrone Siu )

Arrests mount

Police say 4,491 people, aged from 11 to 83, have been arrested since protests began in June.

Demonstrators are angry at what they see as Chinese meddling in Hong Kong's promised freedoms when the then-British colony returned to Chinese rule in 1997.

They say they are responding to excessive use of force by police.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 2 minutes 35 seconds 2 m 35 s Bill Birtles saw police storming the entrance to the university in a "tremendously brutal" operation on Sunday night.

China says it is committed to the "one country, two systems" formula granting Hong Kong autonomy. The city's police denied accusations of brutality and said they showed restraint.

China's ambassador to London on Monday accused foreign countries, including the United States and Britain, of interfering in Chinese internal affairs through their reactions to the violent clashes in Hong Kong.

"Some Western countries have publicly supported extreme, violent offenders," Liu Xiaoming told a London media conference.

He also said Western reporting on Hong Kong was misleading and did not give enough prominence to violence perpetrated by the protesters.

China's Foreign Ministry said on Monday no-one should underestimate its will to protect its sovereignty.

The unrest poses the gravest popular challenge to Chinese President Xi Jinping since he came to power in 2012.

Protesters are seen using a makeshift catapult during unrest in Hong Kong. ( ABC News: Steve Wang )

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