The president of the Baptist University's Student Union, Keith Fong, was arrested on suspicion of possessing an offensive weapon on Tuesday, after police said he was carrying 10 laser pointers in Sham Shui Po.
More than a hundred people gathered outside the district police station to demand his release. Around 11.20pm, officer fired several volleys of tear gas, forcing the crowd to disperse.
Video footage shows four plainclothes officers surrounding Fong and shouting at him, accusing him of trying to escape when they approached him.
Fong says he only tried to run away as he didn't know who they were. He repeatedly asks the officers why he is being held.
Officers later searched his bag and found the pointers. Fong is heard saying he bought them for star-gazing.
After he was arrested, Fong said he felt unwell and was taken to the Caritas Medical Centre.
Representatives from several other universities went to the hospital to see Fong. Speaking to reporters afterwards, they quoted Fong as saying an officer grabbed him by the throat while he was in the ambulance.
The Baptist University Student Union accused the police of abusing their authority and trying to spread fear.