The US National Security Agency eavesdropped on civil rights icon Martin Luther King and heavyweight boxer Muhammad Ali as well as other leading critics of the Vietnam War in a secret program later deemed "disreputable," declassified documents reveal.
The six-year spying program, dubbed "Minaret," had been exposed in the 1970s but the targets of the surveillance had been kept secret until now.
The documents showed the NSA tracked King and his colleague Whitney Young, boxing star Ali, journalists from the New York Times and the Washington Post, and two members of Congress, Senator Frank Church of Idaho and Senator Howard Baker of Tennessee.
The declassified NSA historical account of the episode called the spying "disreputable if not outright illegal".
The documents were published on Wednesday after the government panel overseeing classification ruled in favour of George Washington University researchers who had long sought the release of the secret papers.
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The intensity of anti-war dissent at home led President Lyndon Johnson to ask US intelligence agencies in 1967 to find out if some protests were fuelled by foreign powers.
The NSA worked with other spy agencies to draw up "watch lists" of anti-war critics to tap their overseas phone calls.
The NSA has been accused of overstepping its authority and flouting civil rights protections since the attacks of September 11, 2001.
The agency carried out warrantless wiretapping between 2001-2004 and recent revelations from US intelligence leaker Edward Snowden have exposed far-reaching surveillance of phone records and internet traffic.
The researchers who published the documents on Wednesday said the spying abuses during the Vietnam War era far surpass the current excesses.
"As shocking as the recent revelations about the NSA's domestic eavesdropping have been, there has been no evidence so far of today's signal intelligence corps taking a step like this, to monitor the White House's political enemies," wrote Matthew Aid and William Burr for George Washington University's National Security Archive.
