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.

"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.

The university is at the centre 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 metres 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 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.

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.

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.

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, 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 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.

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.