US authorities secretly tapped the overseas phone calls of prominent critics of the Vietnam war, including Martin Luther King, Muhammad Ali and two actively serving US senators, newly declassified material has revealed.
The National Security Agency (NSA) has been forced to disclose previously secret passages in its own official four-volume history of its cold war snooping activities.
That included tapping into the phone calls and cable communications of two serving senators - Democrat Frank Church and Howard Baker, a Republican who, puzzlingly, was a firm supporter of the war effort in Vietnam.
The NSA also intercepted the foreign communications of prominent journalists such as Tom Wicker of The New York Times and the popular satirical writer for The Washington Post, Art Buchwald, according to the latest revelations.
Alongside King, a second leading civil rights figure, Whitney Young of the National Urban League, was also surreptitiously monitored.
The heavyweight boxing champion, Muhammad Ali, was put on the watch list in about 1967 after he spoke out about Vietnam. He was jailed having refused to be drafted into the army, was stripped of his title and banned from fighting. He is thought to have remained a target of surveillance for the next six years.
The agency went to great lengths to keep its activities, known as Operation Minaret, from public view. All reports generated for Minaret were printed on plain paper without the NSA logo or other identifying markings other than the stamp "for background use only". They were delivered by hand directly to the White House, often going specifically to successive presidents Lyndon Johnson who set the programme up in 1967, and Richard Nixon.
The new disclosures were forced out of the NSA following a call to an appeals panel by the National Security Archive, an independent research institute.