Several protesters have been arrested while trying to run from a Hong Kong university campus surrounded by police.
Around 100 people tried to leave the Polytechnic University, but were met with tear gas and rubber bullets.
In the past week, the campus has turned into a battleground as long-running anti-government protests become more violent.
A small number managed to successfully leave the campus using rope ladders before being picked up by motorcycles.
Hong Kong's Hospital Authority says 116 people have been injured and taken to hospital.
The violence is some of the worst seen during months of unrest in the semi-autonomous Chinese territory. The protests started over a controversial extradition bill, and have now evolved into broader anti-government demonstrations.
China has warned that "no-one should underestimate [its] will to safeguard its sovereignty and Hong Kong's stability", and its ambassador to the UK said the central government would not sit back and watch if the situation became "uncontrollable".
Hong Kong is a part of China, and the protests are, in part, about the fear that the special freedoms the territory enjoys as a former British colony are being eroded.
Earlier, Hong Kong's High Court ruled that a ban on protesters wearing face masks was unconstitutional. The colonial-era emergency law was invoked in October, but protesters largely defied it.
Hong Kong's government said the weekend's events had "reduced the chance" of district elections being held on Sunday as planned, public broadcaster RTHK reports. Postponing or cancelling the vote could further inflame the protests.
The UK has urged an "end to the violence and for all sides to engage in meaningful political dialogue" ahead of the elections.
What is happening?
Police are still besieging the university where several hundred protesters are thought to be trapped. Officers have ordered those inside to drop their weapons and surrender.
A protester inside the university told the BBC supplies, including first aid equipment, were running low.
Meanwhile, a fire broke out on campus and loud explosions were heard, according to the South China Morning Post.
PolyU has been occupied by protesters for several days. On Sunday night, police warned protesters they had until 22:00 (14:00 GMT) to leave the campus, saying they could use live ammunition if the attacks continued.
On Sunday, the university said it had been "severely and extensively vandalised".
A number of protesters left inside in the university have identified themselves as current students in media interviews but it is unclear exactly how many of them are, in fact, university students.
Tears and pride
By Grace Tsoi, BBC News, Hong Kong
Worried parents whose children were trapped inside the Polytechnic University were among the 200 protesters who joined a peaceful rally on Monday night in eastern Tsim Sha Tsui, a tourist area which is only 300 metres away from the besieged campus.
Ms Ng - who only wanted to be identified by her last name - found out on Sunday night her son was among those trapped inside. "He's frightened because he has not faced any emergency situation on his own. She has been on the streets near the university since then.
The teary-eyed mother is proud of her 18-year-old son despite the circumstances. "My son didn't cry. He's strong and likes to help others," she said. "I told my son that you did nothing wrong and you are an outstanding kid. I wouldn't blame you."
She told him to stay inside the campus and wait for her to pick him up. Ms Ng said the government should bear the responsibility for the chaos in Hong Kong.
"Our government is more and more reckless. It ignores the very lowly demands from the citizens!" she said. "I wasn't born in Hong Kong but I love Hong Kong so much! Hong Kong is a wonderful place but it has turned into such a state. It breaks my heart!"
How did we get here?
Campuses remained relatively free of violence during the Hong Kong protests but, last week, the Chinese University of Hong Kong became a battleground.
Police say protesters threw petrol bombs on a major road near the university in an effort to stop traffic. Officers attempted to reclaim the road, leading to major clashes.
The university then cancelled all classes for the rest of the term. Days later, protesters at PolyU also tried to block access to a key tunnel near the university.
Protests have also been held at other locations in Hong Kong.
Why are there protests in Hong Kong?
Hong Kong - a British colony until 1997 - is part of China under a model known as "one country, two systems". Under this model, it has a high degree of autonomy and people have freedoms unseen in mainland China.
The protests started in June after the government planned to pass a bill that would allow suspects to be extradited to mainland China. Many feared this would undermine the city's freedoms and judicial independence.
The bill was eventually withdrawn, but the demonstrations continued, having evolved into a broader protest movement against alleged police brutality, and the way Hong Kong is administered by Beijing.