I watched a video about the legality of security guards doing random compulsory receipt and bag checks and it raised a question for me. So of course they can stop you in the act to prevent an imminent theft but the video never covered the exent of their powers of detention. For instance, you fill a bag with items and run for the door and the security guard intercepts you and prevents you from leaving. You realised you've failed and so drop the bag and go to walk out the door. Can they physically restrain and detain you to hand over to the police can they only protect the property and prevent imminent theft?
A security guard does not have special police privileges, but in the US the shopkeeper has a privilege known as the shopkeeper's privilege to detain a suspected shoplifter for a reasonable time, and the guard can exercise the privilege on behalf of his employer. Depending on the state, that can mean detaining the suspect until the police arrive. This does include reasonable non-deadly force to prevent the suspect from fleeing.
The power of arrest is not limited to police
A person who is not a police officer may, without a warrant, arrest a person if they:
- are in the act of committing an offence
- have just committed an offence
- have committed a serious indictable offence for which they have not been tried
So, in the circumstances you describe where the person has “just committed an offence”, anyone, including a retail security guard can effect an arrest and may use reasonable force to do so.
There is a small exception - members of the armed forces who are on duty may not effect an arrest unless authorised under the Defence Powers Act (which is a whole thing on its own); that’s why military bases are guarded by Federal Police Officers, not soldiers, sailors, or aviators.
If an owner of property or someone authorized by the owner observes another committing a crime in relation to that property, the owner or person authorized can make a citizen's arrest (see Criminal Code, s. 494).
They are then required to call the police and turn the arrested person over to police as soon as possible.
In the UK their power is the same as any other ordinary citizen. In the example you describe, it would be wise to let that person leave and call the police. Detaining them risks breaking the law, once there is no imminent crime taking place. Attempts to restrain them would likely amount to assault.
Generally speaking, unless you are being attacked yourself, you have very little power to restrain other people, and should instead rely on the police.