Video: 'The March That Led to MLK's Arrest and Famous Letter'

(Tuesday, April 16, 1963; during the Birmingham Campaign, part of the Civil Rights Movement) — The Rev. Martin Luther King Jr. wrote his “Letter from Birmingham Jail” today in which the civil rights activist responded to a group of local clergymen who had criticized him for leading street protests; King defended his tactics, writing, “Injustice anywhere is a threat to justice everywhere.”

King was arrested on Apr. 12, 1963, with SCLC activist Ralph Abernathy, ACMHR and SCLC official Fred Shuttlesworth, and other marchers. Thousands of African Americans dressed for Good Friday looked on.

King was met with unusually harsh conditions in the Birmingham jail.

Video: 'Letter from a Birmingham Jail - Martin Luther King Jr.'

An ally smuggled in a newspaper from April 12, which contained “A Call for Unity,” a statement by eight white Alabama clergymen against King and his methods.

The letter provoked King, and he began to write a response to the newspaper. King writes in Why We Can’t Wait: “Begun on the margins of the newspaper in which the statement appeared while I was in jail, the letter was continued on scraps of writing paper supplied by a friendly Black trusty, and concluded on a pad my attorneys were eventually permitted to leave me.”

The letter has been described as “one of the most important historical documents penned by a modern political prisoner,” and is considered a classic document of civil disobedience.