i'm curious as to what kind of situation you stepped into to help someone being attacked and got fucked by the cops?
I'm going to assume that someone was fighting or in danger, you jumped in without thinking and started throwing fists around, cops showed up and booked you for assault. thats not how you go about resloving a heated situation, thats the problem. you dont have to bust ass to help someone in trouble, it's actually more effective to use your brain. You run in there and let everybody know the cops are on there way and that they should stop fighting, and you don't strike unless necessary, instead you defend and retreat when possible leaving the rest to the people who get paid to deal with that shit (po-lice).
hotshot.