The National Security Agency (NSA) of the United States 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 programme later deemed "disreputable," declassified documents have revealed.

The six-year spying programme, 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 documents were published on Wednesday after the government panel overseeing classification ruled in favour of researchers at George Washington University who had long sought the release of the secret papers.

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 programme continued after Richard Nixon entered the White House in 1969, and historians say it reflected a climate of paranoia pervading his presidency.

US Attorney General Elliot Richardson shut down the NSA programme in 1973, just as the Nixon administration was engulfed in scandal.

Recent violations

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 electronic surveillance of phone records and internet traffic.

The researchers who published the documents said the spying abuses during the Vietnam War era far surpass the excesses of the current programme.

"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, a research institute that seeks to check government secrecy.