Power needs an ally to perpetuate itself. From Royal Guard until modern police of today, they always had been to service of power.
Gandhi, Mandela, Luther King, and many other anonymous people were restrained by police. People who fights by civil rights and justice was beaten, shot, gassed, jailed, badly wounded even dead.
Police is a way of repression to the people by their representatives. But power is not only politicians, there is many others interests upper, Capitalism commands. When political persuasion is not enough they need to use force, and many times they used Disproportionate Force.