Quote Originally Posted by David Goldstein View Post
Actually, anytime a police officer stops you to talk to you in his official capacity you are "under arrest", including being talked to about things such as being a witness to a crime. If he takes you into custody that's a different story.
Not true, at least not in the US. He can make a "stop" but not an arrest on "reasonsble suspicion." They generally can't delay you more than 30 minutes or so (varies with jurisdiction) after which you can tell him to arrest you or pound sand, and an arrest requires probable cause. Provable cause legally requires two things: 1) KNOWLEDGE not hunch or suspicion, that a crime has definitely been committed and 2) some reason to connect you with the crime. Absent either if those you can tell him to go away and leave you alone and he has to do so. It's just sense to be polite about it though.