Justification of the level of force used is dependant on the situation at hand and the interaction with the citizenry.
From a philosophical standpoint it could be argued that as regular citizens we have a social contract with the police and our legal system insofar as we have designated that in exchange for a certain level of sovereignty to these authority figures in exchange for maintaining social order and the rule of law. From this perspective we could postulate that a use of force could be designed on a sliding scale from a stern warning from a police office for minor violations on the one side of the spectrum, right through to the use of deadly force for the most sever violations. However that is not to say that police do not act in a manner that goes outside of the boundaries of what is considered sensible in circumstances and to say that there are different viewpoints as to what would be considered adequate. Obviously from one perspective, a person who is being subjected to a degree of force from a police officer is probably going to believe that the use of force that he or she is being subjected to is excessive whereas a non partisan observer might have a completely different objective. As it stands a number of specific mandates are given to police officers at different levels as to what level of force is appropriate in what circumstances. For example it may be the case that the moment a weapon becomes visible the police then have authorization to automatically use less than lethal weapons (Tazers, pepper spray, batons etc.).
From a legal perspective according to US Legal definition (2010) police brutality represents a violation of civil rights when an officer acts with more force than what is required, yet there is no precise definition. In the American context, as a generality, force should only be used to