HONG KONG--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 anti-government 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. In streets nearby, protesters rained down petrol bombs, burning parked cars and the front of a Standard Chartered Bank branch.

Polytechnic University in Hong Kong's Kowloon district 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 lowered themselves several meters from a bridge they had occupied on the campus to a flyover below. They then sped off on the back of motorcycles that were already waiting or arrived quickly.

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

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

The United States expressed concern about the situation but China's ambassador to London accused Western countries of interfering.

FEARS OF BLOODSHED

The size of demonstrations has dwindled in recent weeks, but clashes 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.

Earlier on Monday, police prevented dozens of people breaking out of the university through police lines. Officers had been deployed "on the periphery" of the campus for a week, appealing to "rioters" to leave, police said.

Tsang said there could be bloodshed if the police enter the campus by force and meet strong resistance, adding: "This is something that we want to avoid."

Tsang, who with legal scholar Eric Cheung was the first prominent mediator let onto the campus by police, said it was priority to ensure children trapped inside get out first.

Early on Tuesday, about 20 students accompanied by Tsang left the campus voluntarily, broadcaster RTHK reported on its livestream.

Witnesses estimated there were more than 300 people still on the campus as of late Monday.

CHALLENGE FOR PRESIDENT XI

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.

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

In Washington, Secretary of State Mike Pompeo told reporters the United States was "gravely concerned" about the unrest and violence and said the Hong Kong government bore primary responsibility for ensuring a return to calm.

"Unrest and violence cannot be resolved by law enforcement efforts alone," he said, calling for an independent investigation into protest-related incidents and for the ruling Chinese Communist Party to honour promises of freedom and liberties for the Hong Kong people.

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

"Some Western countries have publicly supported extreme violent offenders," Ambassador Liu Xiaoming told a news conference at which he criticized Western reporting on Hong Kong as "misleading" and not giving enough prominence to violence perpetrated by the protesters.