NSA tapped phone calls of Martin Luther King and Muhammad Ali

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.