Can they detain me downtown for up to 72 hours or something like that? Without technically being arrested or charged with a crime?You dont [sic] have to commit a crime in order to be arrested. The courts determine whether or not you actually committed a crime, not the cops.
However, the cops have to have a very good reason that you may have committed a crime before they can arrest you. To arrest someone in the absence of just cause to believe that the arrestee may have committed a crime is, itself a criminal act.