Signed the Emancipation Proclamation.
…thereby freeing African-Americans from the bondage of slavery.
Note that the Proclamation did not immediately and automatically free slaves in every state and territory. MLK is referring more to the symbolic significance of the document than to its actual, imperfect results.
For more info, see the annotated Emancipation Proclamation on this site.
To help improve the meaning of these lyrics, visit “I Have A Dream” by Martin Luther King, Jr and leave a comment on the lyrics box