Who is the audience for martin luther king, jr's 'letter from a birmingham jail"?

Martin Luther King, Jr's Letter from a Birmingham jail" is mainly addressed to two groups of people, one is his own clergy and the second is the indifferent people of United States. His own group of people has issues with him due to his approach of peace while protesting. The inadvertent message for American people is about bringing the much needed social justice by coming together.