It's a very tough question and not one that's ever really answered formally. The US Constitution has a vague definition but the line has been redrawn and erased so many times.

In theory, it's an easy question: when nobody's natural rights are being violated, police should not intervene. In practice, of course, no police force ever limits themselves to that, so the question becomes - how much the citizens would agree to tolerate and under which sauce? Experience shows, that the first is "a lot" and the second is "depending on how much you scare them, if you are good at scaring, you can pull off practically anything".