Declassified files have revealed that the National Security Agency in the US spied on Martin Luther King and Muhammad Ali during the 1960s and 70s.
The "eye-popping" documents show that the NSA tapped the overseas communications of a number of prominent figures during the Vietnam War.
The NSA operation "Minaret" was first exposed in the 1970s, but until now the names of those on the "watch list" had been kept secret. One official later described the project as "disreputable if not outright illegal".
Minaret was designed to watch for potential threats to the president, for drug dealers and "domestic terrorism". President Lyndon Johnson had wanted to know if domestic anti-war leaders and organisations were backed by foreign powers.
Martin Luther King became an NSA target for being an outspoken critic of the war. He was the subject of FBI wiretaps over allegations of connections with the Communist party.
Another high-profile NSA target was Urban League president Whitney Young, who also opposed the war. In 1969, he said: "[The war is] tragically diverting America's attention from its primary problem - the urban and racial crisis - at the very time that crisis is at [a] flashpoint."
New York Times bureau chief Tom Wicker and Washington Post humorous columnist Art Buchwald were both placed on the list for their anti-war opinions, as was Sen Frank Church, who criticised Johnson's decision to escalate the Vietnam War in 1965.
Boxing champion Muhammad Ali, then known as Cassius Clay, was publicly against the war and avoided draft on the grounds that he was a Muslim minister. He said: "I've got nothing against them Vietcongs. I can fight in wars declared only by Allah himself." His appeal was rejected and he was sentenced to five years in prison for refusing to serve.
Files released by the Interagency Security Classification Appeals Panel (Iscap) also show surveillance that had international implications.
An interception from August 1961 provided an advance warning of the decision to close Berlin's internal borders, which eventually led to the erection of the Berlin Wall, splitting the communist-run section from those run by Western powers.
The NSA was the only intelligence agency collecting information about East Germany at the time. A party message revealed plans to start blocking all foot traffic between East and West Berlin.
Files also show that the NSA also played a key role in the Soviet missile crisis in Cuba in the 1960s and in the 1977 Panama Canal Treaty. The agency had placed surveillance on Panama's president Omar Torrijos for years.
These revelations come as the NSA was placed under further scrutiny regarding its domestic surveillance programmes and data swoops on allied countries in the wake of Edward Snowden's exposure of agency methods.
Researchers Matthew Aid and William Burr said of the declassified files: "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."