I performed a citizen’s arrest once (a VERY drunk driver who was broken down on the side of the Interstate and was trying to drive away). A Nevada Highway Patrol officer finally showed up, cuffed him, put him in the back of his car, and suggested the citizen’s arrest since he hadn’t witnessed the suspect’s activity but I had (long, rather harrowing story). He said all I had to do was look in the direction of the guy (a good 20 feet away inside a closed car) and say “you’re under arrest”. I did so. He was later convicted.
Wouldn’t have happened in Britain. In Britain you actually have to witness an indictable offence (roughly equivalent to a felony) in order to be able to arrest someone. And you have to inform them exactly why you are arresting them. You would need witnesses to back you up when you say you saw the offence committed, and God help you if you injure them in the process.
A sworn constable on the other hand (i.e. a police officer) only has to suspect someone of committing an offence (whether summary (misdemeanor) or indictable in order to effect an arrest, and as long as they don’t screw up the arrest or the reasons for it too egregiously, they will have a lot more legal protection for not doing it correctly (although the police constabulary as a body might be held civilly liable and the officer’s boss might go apes**t at the officer for screwing it up).
Hm... I wonder if this could be argued in Zimmerman’s trial.