The alleged victim was also arrested, on suspicion of possessing child pornography.
Police linked the arrests to a video which circulated widely on social media and appeared to show a group of men ganging up on a man and accusing him of asking underage boys for sex.
The man in the video was ordered to kneel down in the street, and after grabbing his mobile, the men read out messages purportedly on it and demanded details of the man’s relatives.
Police said the 12-minute video was made in Kowloon City last month. It received more than 434,000 views on YouTube.