I wonder how far we are away from people being arrested for holding up blank pieces of paper.
It’s called a Kavanaugh Stop. https://en.wikipedia.org/wiki/Kavanaugh_stop
Detaining is not something I really understanding, but from what I have read it is using restraint of some sort without arrest and not really something police seem to have any special powers to do (whereas their powers of arrest are greater than those of other people).