What's the difference between a policeman and a security guard?

Nowadays, many security guards' clothes are imitations of police uniforms, which you probably can't tell clearly. As long as you are careful, it is not difficult to distinguish the real police uniform. On the right chest of the police uniform, there is a metal police identification card with the words "territorial" or "judicial" written on it. For example, the public security in Beijing is written as "Beijing". If it is "judicial", it means judicial police; The left chest is a warning sign, and the left arm has an armband with the words "police" on it.