Kinda of. As a photographer I always have my DSLR with me, and I would always film of photograph anything I felt was wrong (Even though I believe that's actually illegal over here now)
I have only intervened to stop brutality on one occasional. My partner was being sectioned because of her mental health problems, and while it was not an arrest (Though they could have arrested her if they'd wanted too) she did have to be cuffed. She resisted the cuffs, and tried to get away, so it all had to get a little physical: And I felt that there use of power was over kill and consisted of brutality.
They were hurting her far too much in their attempts to get the cuffs on her, so I threw one of them off and tried to take care of it myself. I of course got thrown back again, but after I tried it a second time, they left me to it. I was able to calm her down, and agree for the officers to put the cuffs on her. By intervening I was able to stop someone from getting hurt (cos she would not have stopped until someone was seriously hurt).
However, in all fairness to the officers, while I feel they did use overkill, both of us could have been prosecuted for assaulting them, but they didn't do so: I assume because they knew she was too ill to control herself, and they realized that I knew I could do a better job of getting the cuffs on (Ie: Do it non violently) than they could, and threw them off to prevent anyone getting hurt. I later phoned the station to leave my thanks, as I have never dealt with police officers who were so good before.
If I ever see anyone who is being put in a unnecessary amount of pain by the police, I will always intervene, even if I am hopelessly outnumbered. I always stand up for what I believe is correct.