On August 28th 1963, the American civil rights leader, Martin Luther King, made his historic plea for an end to racial discrimination in the USA. Standing on the steps of the Lincoln Memorial, he addressed hundreds of thousands of activists who had marched to Washington to demonstrate for black rights.

Listen to John Lewis, the youngest speaker on the podium that day.

Photo: Associated Press.

In 1967, the world heavyweight champion, Muhammad Ali, refused to be indicted into the American military. His decision to follow his conscience and not serve in Vietnam galvanised radicals across the US.

Simon Watts speaks to Dr Nathan Hare about a visit by Muhammad Ali to Howard University at the height of the outcry over his refusal of the draft.

(Photo: Muhammad Ali in training. Credit: R McPhedran/Express/Getty Images)

The soul singer's April 1968 concert was held amid rioting and violence provoked by the assassination of Martin Luther King. But despite the fears of the city authorities, the streets of Boston were quiet the night James Brown and his band played. Listen to two people who were there.

(Photo: James Brown. Credit: AFP)

For nearly 40 years, the US government conducted an experiment on a group of African-American men without their knowledge - to see what would happen if their syphilis was left untreated.

Photo: US National Archive.