Hong Kong leader Carrie Lam told a news conference on Tuesday that about 100 protesters remained inside the Hong Kong Polytechnic University campus that has been sealed off by police.
Lam, whose government has been a target of demonstrations over recent months, also said she had told police to handle the protesters that remain in the building humanely.
But she said that the police would take "neccessary action" if protesters reacted violently to police, and said that the campus had been turned into a "weapons factory".
Earlier, dozens of protesters attempted a dramatic escape from the university campus by shimmying down plastic hosing from a bridge and fleeing on motorbikes as the police fired projectiles.
Polytechnic University in Hong Kong's Kowloon district 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.
Lam said that 600 protesters in total had left the campus by Tuesday, incuding 200 below the age of 18.
On Monday, two prominent figures were allowed by police onto the campus to mediate, a sign that there was 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, said 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 a bank.
Some of the protesters who escaped on Monday 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.
US, EU condemn violence
The European Union and the United States have condemned the escalating violence in Hong Kong amid fears of a bloody crackdown.
"The Hong Kong government bears primary responsibility for bringing calm to Hong Kong," US State Secretary Mike Pompeo said. "Unrest and violence cannot be resolved by law enforcement efforts alone. The government must take clear steps to address public concern."
The EU joined the condemnation with a spokeswoman for foreign affairs at the European Commission expressing "deep concern" on Monday over reports that Hong Kong first responders and medical staff were being detained by law enforcement forces, preventing them from providing assistance to injured people.
"Any violence is of course unacceptable and any action by the law enforcement authorities must remain strictly proportionate and fundamental freedoms, including in particular the right of peaceful assembly and expression, must be upheld," Maja Kocijancic told reporters.
Britain also described itself as "seriously concerned" over the violence on Monday with a spokesperson for Prime Minister Boris Johnson saying London continues to urge "restraint on all sides and support the right to peaceful protest."
The Foreign Office added that "it is vital that those who are injured are able to receive appropriate medical treatment, and that safe passage is made available for all those who wish to leave the area."
Speaking to Euronews, pro-democracy Hong Kong politician Emily Lau asked for a stronger response from the international community, warning a humanitarian crisis is brewing.
"It seems the police are very very tough and they will not let the people come out. And there are hundreds of them trapped there (in the university), and some are injured, and there may not be enough food or water, so it's going to turn out to be a humanitarian crisis, and we don't need that in Hong Kong," she said.
"Many people could be killed, there could be a massacre, like Tiananmen Square, 30 years ago," Lau added.
She also called for the UK to give passports to Hong Kong inhabitants in need of protection.
China's ambassador to London accused western countries 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 criticised Western reporting on Hong Kong as "misleading" and not giving enough prominence to violence perpetrated by the protesters.
'We need help'
According to Hong Kong's Hospital Authority, 38 people were wounded during the night of Sunday to Monday.
Dan, a 19-year-old protester on the Polytechnic University campus, said protesters may need international help."
"We've been trapped here for too long. We need all Hong Kongers to know we need help," he added, bursting into tears. "I don't know how much longer we can go on like this."
Police, who have faced an array of weapons including petrol bombs, bow and arrows and catapults, urged protesters to leave.
"Police appeal to everyone inside the Polytechnic University to drop their weapons and dangerous items, remove their gas masks and leave via the top level of Cheong Wan Road South Bridge in an orderly manner," they said in a statement.
One country, two systems
Recent days have seen a dramatic escalation of the unrest that has plunged the Asian financial hub into chaos for almost six months.
Demonstrators angry at what they see as Chinese meddling in Hong Kong's promised freedoms when it 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, with the city's police accusations they use undue violence.
Separately, Hong Kong's High Court ruled on Monday that a British colonial-era emergency law revived by the government to ban protesters wearing face masks was unconstitutional.
It said the law was "incompatible with the Basic Law", the mini-constitution under which Hong Kong returned to Chinese rule in 1997.