The Bloody Sunday.

Between 1961 and 1964, the Student Nonviolent Coordinating Committee (SNCC) had led a voting registration campaign in Selma, a small town in Dallas County, Alabama, with a record of consistent resistance to black voting. When SNCC’s efforts were frustrated by hard resistance from the county law enforcement officials, Martin Luther King, Jr. and the Southern Christian Leadership Conference (SCLC) were persuaded by local activists to make Selma’s intransigence to black voting a national concern. SCLC also hoped to use the momentum of the 1964 Civil Rights Act to win federal protection for a voting rights statute. During January and February, 1965, King and SCLC led a series of demonstrations to the Dallas County Courthouse.


On February 17, protester Jimmy Lee Jackson was fatally shot by an Alabama state trooper.  In response, a protest march from Selma to Montgomery was scheduled, so on March 7, 1965 six hundred people assembled at a downtown church, knelt briefly in prayer, and began walking silently through the city streets. They were led by John Lewis and other SNCC and SCLC activists, crossed the Edmund Pettus Bridge over the Alabama River to Montgomery. 


But the marchers were stopped as they were leaving Selma, at the end of the Edmund Pettus Bridge, by some 150 Alabama state troopers, sheriff’s deputies, and posse men, who ordered the demonstrators to disperse. One minute after a two-minute warning was announced, the troops advanced, wielding clubs, bullwhips, and tear gas. John Lewis, who suffered a skull fracture, was one of fifty-eight people treated for injuries at the local hospital. The day is remembered in history as “Bloody Sunday”.


For more information I'll leave a link of a History Channel's documentary.
- Bloody Sunday (1965)