If you or someone nearby is a victim of obvious police brutality, can you fight back?

You fight back by suing. If you beat the crap out of the cop, you will be beaten up by the entire police force, and then jailed. You gather evidence and you sue. It is a civil rights violation as well as an assault and battery.

No, a police officer can only use reasonable force to carry out their duties. But in general, if an officer is using unreasonable force it is more practical to do nothing; the police carry guns, some times tasers, a billy club, and can quickly call for backup. You are going to lose in any fight with the police, whether on your behalf or someone else's. It is better not to resist, try to record what went on, and then complain to the Department's Internal Affairs Division and perhaps the press.

You ought to be smart enough not to get into a fight with police. They often will lie to protect themselves. Other police will lie to protect them. Why don't you choose to play elsewhere you cant complain. You can take them to court but you probably wont fare very well.

You cannot interfere with the police by physical force. You can tell them verbally that they are committing abuse and to stop and you can film it. As for being personally assaulted yourself, yes you can defend yourself, but as you stated the result will be that they charge you with resisting or worse. Plus, if you fight back, they come on even harder and they have tazers, clubs, dogs and guns.

I will give the answer under the law, and the answer under the real world. First, I will rephrase our question. You say if you have little or no reason to be assaulted. If the officer has a legal reason, little or large, to use force, it changes the answer. No an officer can only use that degree of force which is necessary under the circumstances. For example, an arrest necessitates some force, if only what is required to but on handcuffs and take you to jail. If one resists arrest, the officer can use reasonable force to subdue the arrestee. However, once the arrestee is under control, and officer cannot further assault the arrestee. If an officer assaults someone beyond the reasonable degree of force that is needed to subdue an arrestee, or for the officer to defend himself if he is attacked, then, yes, you can fight back, and a third party can step in t prevent the assault. In the real world, if your defense to a crime comes down to your word against an officer's word, and you have to convince the court that the officer is lying, then you lose. Thus, if an officer uses excessive force, and you fight back, or even if you do not fight back but the officer lies and says you resisted arrest and he was just defending himself, and you have no witnesses but yourself, you will likely lose in court, even though the officer would actually be guilty of assault and battery. If you use a weapon to defend yourself, then that could increase our chances of going to prison. No one would ever have heard of Rodney King if the assault in that case had not been caught on video. It is illegal for police to use excessive force. However, they do on occasion violate this law and it is difficult to do anything about it.