Is it legal to video tape a police officer as he/she deals with my friend or strangers?

O.k, usually at cinco de mayo i see some pretty aggressive behavior from cops to pedestrians. a couple of years ago i saw a mexican guy get trampled by a police man on horse downtown. the ambulance came and everything, but it was never on the news. my best friend constantly gets racially harrased by the police, and i say racially beaocuse they usually end up saying things like "now get your black *** out of here", "i'll never belive a word that comes out of you ****** mouth" and a sargent at one point pointed at his stripes and said "you see these here? that means that i can do what i want" while doing an un warented search. A couple of times my friend has called me down to be a witness doing these unsolicited searches, and i think it's about time this was on tape. are there any laws against videotaping a police stop/search? i know they carry cameras in their cars for their protection, am i allowed to carry one for mine? thanks, i also want to video tape strangers getting harrased.