From the FAQ on the official website of the CPUSA:
Why does the Communist Party oppose Violence?
Communists believe that social change can only be accomplished through the united action of mass movements which express the majority will of the people. Peaceful methods of change are not only the right thing to do, they are the most effective way to unite and mobilize the greatest majorities.
Violence, on the other hand, is a tool of the big corporations and the governments they control. To preserve their power, they use violence against workers’ and people’s movements.
In contrast, Communists seek to change society peacefully. We work to expand every democratic and electoral avenue as part of our fight for working class political and economic power.
Our party believes that it is possible to make fundamental transformations using the electoral process, the Constitution and especially the Bill of Rights.
Marx on the dictatorship of the proletariat:
“Political power … is merely the organized power of one [socio-economic] class for oppressing the other.”
“[The working class] must act in such a manner that the revolutionary excitement does not collapse immediately after the victory. On the contrary, they must maintain it as long as possible. Far from opposing so-called excesses, such as sacrificing to popular revenge of hated individuals or public buildings to which hateful memories are attached, such deeds must not only be tolerated, but their direction must be taken in hand, for examples’ sake.”
“Communists disdain to conceal their views and aims. They openly declare that their ends can only be attained by the forcible overthrow of all existing social conditions (e.g. bourgeois democracy).”
“The arming of the whole proletariat with rifles, guns, and ammunition should be carried out at once [and] the workers must … organize themselves into an independent guard, with their own chiefs and general staff. … [The aim is] that the bourgeois democratic Government not only immediately loses all backing among the workers, but from the commencement finds itself under the supervision and threats of authorities behind whom stands the entire mass of the working class. …As soon as the new Government is established they will commence to fight the workers. In order that this party (i.e., the democrats) whose betrayal of the workers will begin with the first hour of victory, should be frustrated in its nefarious work, it is necessary to organize and arm the proletariat.”
“But the anti-authoritarians demand that the political state be abolished at one stroke, even before the social conditions that gave birth to it have been destroyed. They demand that the first act of the social revolution shall be the abolition of authority. Have these gentlemen ever seen a revolution? A revolution is certainly the most authoritarian thing there is; it is the act whereby one part of the population imposes its will upon the other part by means of rifles, bayonets and cannon — authoritarian means, if such there be at all; and if the victorious party does not want to have fought in vain, it must maintain this rule by means of the terror which its arms inspire in the reactionists. Would the Paris Commune have lasted a single day if it had not made use of this authority of the armed people against the bourgeois? Should we not, on the contrary, reproach it for not having used it freely enough?”
“It is natural for a liberal to speak of “democracy” in general; but a Marxist will never forget to ask: ‘for what class?’ Everyone knows, for instance (and Kautsky the ‘historian’ knows it too), that rebellions, or even strong ferment, among the slaves in ancient times at once revealed the fact that the ancient state was essentially a dictatorship of the slave owners. Did this dictatorship abolish democracy among, and for, the slaveowners? Everyhody knows that it did not.”
“Dictatorship is rule based directly upon force and unrestricted by any laws. The revolutionary dictatorship of the proletariat is rule won and maintained by the use of violence by the proletariat against the bourgeoisie, rule that is unrestricted by any laws.”