One protection that everyone has in this country is the right to not have to endure cruel and unusual punishment. This is provided by the Eighth Amendment of the Constitution. While there is some question about exactly what constitutes this forbidden punishment, there are some rulings and cases that shed some light on what shouldn't happen.
Police officers are held to a high standard when they are on duty. There are limits to what types of force officers can use when dealing with people. For example, a cop can't take his gun off his belt and hit you across the face just because you don't want to give a confession. Other situations are also covered.