HONG KONG (Reuters) - Dozens of Hong Kong protesters staged a dramatic escape from a university campus sealed off by police on Monday by shimmying down plastic hosing from a bridge and fleeing on waiting motorbikes as the police fired projectiles.

Many more protesters remained trapped inside the Hong Kong Polytechnic University, and two prominent figures were allowed by police onto the campus late on Monday to mediate, a sign that there is a growing risk of bloodshed.

"The situation is getting more and more dangerous," Jasper Tsang, a pro-Beijing politician who is the former head of Hong Kong's Legislative Council, told 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. 

The protests that started in June over a now-shelved extradition bill have snowballed into an anti-China campaign amid anger over what many view as Beijing's interference in Hong Kong's autonomy since the former British colony was returned to Chinese rule in 1997.

(Pictured) Protesters are detained by police near the Hong Kong Polytechnic University on Nov. 18. Pro-democracy demonstrators holed up in a Hong Kong university campus set the main entrance ablaze November 18 to prevent surrounding police moving in, after officers warned they may use live rounds if confronted by deadly weapons.

The university is at the center of a standoff in the past week that has seen the most intense violence in five months of anti-government demonstrations.

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

© Reuters/ADNAN ABIDI Anti-government demonstrators take cover during clashes with police near the Hong Kong Polytechnic University (PolyU) in Hong Kong

Other protesters, hurling petrol bombs, tried repeatedly to break into the campus but police fired tear gas and water cannon Monday 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 serious condition.

TIGHTENED CORDON

Earlier on Monday, police 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 and stop them from fleeing.

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’re afraid we may see bloodshed. This is something that we want to avoid," Tsang said.

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 the campus 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, appealing to "rioters" to leave.

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

Witnesses estimated there were more than 300 people still on the campus. 

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.

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

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. Beijing denies interfering in Hong Kong's affairs and has blamed Western countries for stirring up unrest.

The Hong Kong government invoked a colonial-era emergency law in October banning faced masks commonly used by protesters. The High Court ruled on Monday the ban was unconstitutional and police said they would suspend all such prosecutions.

(Reporting by Marius Zaharia, James Pomfret, Josh Smith, Jessie Pang, Joyce Zhou, Donny Kwok, Anne Marie Roantree, Twinnie Siu, Greg Torode, Kate Lamb, Farah Master, Jennifer Hughes and Tom Lasseter in Hong Kong and Phil Stewart in Bangkok; Writing by Greg Torode and Tony Munroe; Editing by Stephen Coates, Robert Birsel and Timothy Heritage)