Question

"Do citizens have the right to protest a law they feel is an injustice?"

Answer 1

Yes they do. We have freedom of speech which is stated in the First Admendment.

Answer 2

Yes citizens have the right to protest a law they feel is injustice.  

No human rights instrument or national constitution grants a direct right to protest, but such a right is the manifestation of the right to freedom, the right to freedom of assemble, and the right to freedom of speech which protects the ability to express protest and to wear bands and arrange peaceful marches and protests on certain public lands.