If a police officer is abusing their authority, you have to prove it. You can ask that the actions of the officer be investigated for the possibility of abuse. Since a lot of people are arrested, but struggle in the process, what the officer has to do to restrain the person gets called abuse. So, you need to have proof, like the officer says stop, and you stop. Then the officer takes out his night stick and beats you with it. You did stop, and made no move to resist arrest. If you can prove this, or get it investigated, then the officer is in a lot of trouble. If you are being attacked by an officer, the first question that has to be asked is, did you follow their order to stop what you were doing, and calmly go with them. If not, then they might very well have justifiable cause to beat you into submission. In this case, the sooner you stop, the sooner they will. If another person assaults you, yes, you have the right to defend yourself. If you feel that the other person is going to kill you, you have a right to self defense. The rub is, were they out to kill you or not. You might have to show just cause for your action. That, however does not stop other legal actions against you if the persons family decides to sue you.