Police protect their own first and foremost, whether their comrades are right or wrong. Therefore, if you intend to exercise your legal right to photograph them, I suggest doing so with a telephoto lens from a position that conceals your presence. Once they are aware of your presence, you will be perceived as a nuisance at best, but more likely, a potential threat. Expect harsh words and be prepared to comply with unconstitutional demands or face unjust repercussions. Do not expect to educate them on the legality of photography in public places unless you enjoy the tingle of 50,000 volt pulses....amateur photographs and films are proving to be an important tool in fighting crime - including crime committed under colour of authority.