I’ve seen a few references to the concept of citizen’s arrests lately, which motivated me to post some of the things I’ve discovered. The short answer is this: you should never, ever attempt a citizen’s arrest.
It’s first worth mentioning that merchants get additional protection under the shopkeeper’s privilege. I’d give the same advice, though: except for large department stores which have excellent lawyers and are surely much more intricately familiar with the law than I am, you should never, ever attempt a citizen’s arrest.
The police have explained the issue as a matter of safety: if some crazy guy goes around shooting people and you try to detain him, it will most assuredly end badly for you. However, removing the element of personal safety, I’d argue that even if you have been personally wronged, a detainment is a bad idea.
While the Shopkeeper’s Privilege seems to suggest you just have to go by reasonable suspicion, my understanding of the law regarding citizen arrests (really a detainment) are that the crime must have been “in fact committed.” So enter nightmare scenario #1: you suspect someone has just robbed the bank and detain them. It turns out that they were simply wearing their hamburgler mask and had withdrawn their cash from the bank, and then went for a job. What do you get for your valiant attempt at stopping crime? Arrested, of course, for unlawful imprisonment, kidnapping, or a related charge.
In most places, it is only permitted in regard to felonies and/or crimes you personally witnessed. Scenario #2, you see a person unloading lots of merchandise into their car that was tucked under their shirt, and you detain them. Not a felony (kind of), and you didn’t witness the shoplifting occurring. You’re probably getting arrested.
And these are all criminal wrongs. You’re also just asking to get sued in civil court, too. (I wonder… If someone attempts to effect a citizen’s arrest which you believe to be improper, can you then place them under a citizen’s arrest, on the grounds that they’re committing kidnapping / etc.?)
And as something totally off-topic, the police don’t have to “read you your rights” (the Miranda warning) unless you are (1) in their custody and (2) being questioned about the crime. In the citizen’s police academy, one of the officers mentioned that it was pretty amusing how often people would act all smug, thinking their case would be dismissed because they weren’t read their Miranda rights at the time of their arrest.