Protesters inside Polytechnic University are said to be running low on supplies, with one lawmaker saying they could not last "another day".
Those inside expect to be arrested and charged with rioting if they walk out.
The university has turned into a battleground as anti-government protests show no signs of slowing down.
Tensions could be further inflamed by China condemning a decision by Hong Kong's high court to overturn a ban on face-masks.
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.
Police later moved in, surrounding the campus, leading to protesters throwing petrol bombs and firing rocks from catapults.
BDST: 1150 HRS, NOV 19, 2019