At what point is civil disobedience justified in dealing with a corrupt government?

Suppose the government of a free and sovereign nation was taken over by a secret society that wanted to take away their freedoms and destroy their constitution.

Suppose further that the secret society had infiltrated every level of the government and correcting the problem through the political process was no longer an option.

Is it moral and ethical for the citizenry to turn to civil strife and warfare in overthrowing the scoundrels? If so, at what point is it appropriate to take action? How would the freedom loving citizens best go about overthrowing the corrupt government?