Attempts by Congress to extend the powers of the Freemen's Bureau was vetoed by President Andrew Johnson in February, 1866. In April 1866, Johnson also vetoed the Civil Rights Bill that was designed to protect freed slaves from Southern Black Codes (laws that placed severe restrictions on freed slaves such as prohibiting their right to vote, forbidding them to sit on juries, limiting their right to testify against white men, carrying weapons in public places and working in certain occupations).
The first branch of the Ku Klux Klan was established in Pulaski, Tennessee, in May, 1866. A year later a general organization of local Klans was established in Nashville in April, 1867. Most of the leaders were former members of the Confederate Army and the first Grand Wizard was Nathan Forrest, an outstanding general during the American Civil War. During the next two years Klansmen wearing masks, white cardboard hats and draped in white sheets, tortured and killed black Americans and sympathetic whites. Immigrants, who they blamed for the election of Radical Republicans, were also targets of their hatred. Between 1868 and 1870 the Ku Klux Klan played an important role in restoring white rule in North Carolina, Tennessee and Georgia.
You will find more information about these related subjects here:
Slavery in the United States
The American Civil War
Civil Rights Movement