Comment: I'll just add that they can't arbitrarily detain you if you're just walking down the street. In that case, the interaction is voluntary and you can ask "Am I free to go?" and if they don't answer "No, you are under arrest/being detained", then you are perfectly within your rights to blow them off and continue on your way. Of course, they might fabricate a story and give you a beat-down anyway, but that's another issue.