When can I legally arrest someone?
In Hong Kong, there is a law that empowers any person to arrest another person under circumstances described by the law.
Chapter 221: Criminal Procedure Ordinance
The key word in this section is "arrestable offence". That means you cannot arrest whenever you suspect any offence.
What is an arrestable offence?
Arrestable offence is defined under Chapter 1: Interpretation and General Clauses Ordinance.
The bottom lines:
1) Use the least amount of force necessary
2) Call police as soon as possible
3) The spirit of the law is to prevent crime or minimise damage dealt by the crime.