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Seven other defendants, including the former secretary-general of the banned African National Congress (ANC), Walter Sisulu, were also given life prison sentences.
Crowds gathered silently outside the court building in Pretoria's Church Square waiting for the verdict to be handed down. Hundreds of police patrolled the area.
The Rivonia trial - named after the suburb of Johannesburg where several of the defendants were arrested - began eight months ago, with Mandela, 46, and his co-defendants proudly confessing their guilt to plotting to destroy the South African state by sabotage.
As members of the ANC - the main African nationalist movement - they have campaigned for an end to the oppression of black South Africans.
But the movement was banned in 1960 following the Sharpeville massacre and campaigners decided they had no choice but to resort to violent means.
Struggle for equal rights
Mandela - a lawyer by training - told the court earlier: "I do not deny that I planned sabotage. I did not plan it in a spirit of recklessness nor because I have any love of violence. I planned it as a result of a calm and sober assessment of the political situation that had arisen after many years of tyranny, exploitation and oppression of my people by the whites."
His co-accused included: Walter Sisulu, Dennis Goldberg, Govan Mbeki, Raymond Mhlaba, Elias Mosoaledi, Andrew Mlangeni - all ANC officials and Ahmed Kathrada, the former leader of the South African Indian Congress.
Lawyer for the defendants, Harold Hansen QC said: "These accused represent the struggle of their people for equal rights. Their views represent the struggle of the African people for the attainment of equal rights for all races in this country."
But the judge, President Quartus de Wet, said he was not convinced by their claim to have been motivated by a desire to alleviate the grievances of the African people in this country.
Judge de Wet said: "People who organise revolution usually plan to take over the government as well through personal ambition."
However, he stopped short of the imposing the supreme penalty of death.
The convicted men were cheered as they left court in a police lorry. The crowd was dispersed without any serious incident.
This was the second time Nelson Mandela had been tried for high treason - in 1956 he was charged but after a four year trial the case was dropped.
There were demonstrations in Britain following the 1964 sentencing. A world petition calling for the prisoners' release was handed to the United Nations Secretary General.
Nelson Mandela spent most of his 27 years behind bars serving hard labour in Robben Island prison off Cape Town.
He was released in 1990, jointly awarded the Nobel peace prize with President FW de Klerk in 1993 and elected South Africa's president in the country's first multi-racial elections held in 1994.
He stepped down in favour of Govan Mbeki's son Thabo in 1999 but continues to travel the world campaigning for peace.
Walter Sisulu died at the age of 90 in May 2003.
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