Those who are still inside Polytechnic University (PolyU) are said to be running low on supplies.
Adults who leave face arrest, and some are too scared to come out.
Hundreds of protesters tried to run from the campus on Monday, but many were hit with tear gas and rubber bullets and arrested.
Police have revealed that on Monday alone, at the campus, they used:
- 1,458 tear gas canisters
- 1,391 rubber bullets
- 325 bean bag rounds
- 265 sponge bullets
A small group of protesters managed to leave using rope ladders before being picked up and driven away by motorcyclists.
On Sunday night, police warned protesters they had until 22:00 (14:00 GMT) to leave the campus.
The violence at PolyU is one of the biggest flare-ups Hong Kong has seen since protests broke out in June.
Tuesday saw the UN human rights office urge the Hong Kong authorities to de-escalate the situation at PolyU and address the humanitarian situation of those inside which it said was “clearly deteriorating”.
It added that some demonstrators’ use of violence, including against the police, “cannot be condoned”.
But underpinning it all is the fear Hong Kong’s unique identity is threatened by China.
Tensions now could be further inflamed after China condemned a decision by Hong Kong’s high court to overturn a ban on facemasks.
What is happening on Tuesday?
An estimated 100 to 200 protesters still remain in PolyU, authorities have said.
One protester called Jeff told the BBC that he was hiding in a building on the campus.
“I have been trapped inside Polytechnic University for two days and food and water is running out,” he said.
“Most people tried to get out of this hell-like place, most of them tried to escape but ended in failure, being arrested or wounded or something.”
“I didn’t expect this incident to get so desperate in here. The campus is so desperate and I am not well prepared. I would call it a disaster.”
He said his parents were concerned for his safety and he had told them he would make it out.
One tourist from mainland China is too scared to leave. He told BBC News Chinese that he was curious about what was going on at the university and had arrived on Sunday at the invitation of a friend who studies there.
After a visit to the library and a meal, he learnt that police had cordoned off the whole area.
“I am not a rioter,” the tourist, who wishes to stay anonymous said. “I don’t want to get caught and I just need to go back to China.”
A handful of protesters have been trickling out of the university, some suffering from hypothermia and leg injuries, according to news site SCMP.
One protester said he had decided to come out because of “hunger and cold”, adding that many inside were “hurt without enough medical supplies”.
Another 16-year-old protester told Reuters new agency she had chosen to “surrender”.
“We have been trying to escape since yesterday morning. But then we couldn’t find a way out [and] were afraid of being charged,” she said. “This is the only way… I was quite desperate.”
In the early hours of Tuesday morning, more than 200 students below the age of 18 left, accompanied by education officials including high school principals.
Those under the age of 18 had their identities recorded and were let go. Adults were arrested.
ong Kong Chief Executive Carrie Lam called on all protesters to surrender, saying no violence would happen if they came “out in a peaceful manner”.
However, she said police would have to take “necessary action” if that changed. Separately on Tuesday, Hong Kong’s new police chief took office.
Chris Tang said the force was not able to end the protests alone, saying the unrest would only end if society condemned the violence.
A number of people have supported protesters behind the scenes, donating supplies to them or picking them up from protests and driving them home to avoid arrest.