The Washington-based group Property of the People released the papers to mark the 100th anniversary of Mandela's birth. It said it obtained them after years of litigation.

"The documents reveal that, just as it did in the 1950s and 60s with Martin Luther King Jr and the civil rights movement, the FBI aggressively investigated the US and South African anti-apartheid movements as Communist plots imperiling American security," the group's president Ryan Shapiro said in a statement.

"Worse still, the documents demonstrate the FBI continued its wrong-headed Communist menace investigations of Mandela and the anti-apartheid movement even after US imposition of trade sanctions against apartheid South Africa, after Mandela's globally-celebrated release from prison, and after the fall of the Berlin Wall."

South Africa's first black president, who died in 2013 and remains a global icon for his struggle against apartheid and message of reconciliation after 27 years in prison, was regarded with suspicion by Washington during the Cold War and remained on the US terrorism watchlist until 2008.

Property of the People said its trove included documents from the major US intelligence agencies, the FBI, CIA, DIA and NSA, most of which have never been seen by the public.

"The Mandela Files" can be found on its website https://propertyofthepeople.org/.

Mandela's African National Congress (ANC) has been in power since the end of apartheid in 1994 and remains in a governing coalition with the South African Communist Party, which also resisted the white-minority government.

Southern Africa was a key Cold War battleground, as newly independent states in the region such as Angola and Mozambique aligned with Moscow.

Celebrations have been held across South Africa this week to mark Mandela's 100th birthday, including a rousing speech on Tuesday by former US president Barack Obama, who said the world should resist cynicism over the rise of strongmen.

Australian Associated Press