JOHANNESBURG - Celebrated South African anti-apartheid activist Ahmed Kathrada, a Robben Island prisoner and one of Nelson Mandela's closest colleagues in the struggle against white rule, died Tuesday aged 87.

Kathrada was among those tried and jailed alongside Mandela in the Rivonia trial in 1964, which drew worldwide attention to the brutalities of the apartheid regime. He died in hospital in Johannesburg after a short illness following brain surgery, his charity foundation said.

Kathrada spent 26 years and three months in prison, 18 of which were on Robben Island, the notorious jail off the coast of Cape Town. After the end of apartheid, he served from 1994 to 1999 as parliamentary counsellor to President Mandela in the first African National Congress (ANC) government. Leading a flood of tributes, retired Archbishop Desmond Tutu described Kathrada as "a man of remarkable gentleness, modesty and steadfastness", hailing him a moral leader of the anti-apartheid movement.

"These were people of the highest integrity and moral fibre who, through their humility and humanity, inspired our collective self-worth - and the world's confidence in us," Tutu said in a statement.

Kathrada's activism against white-minority apartheid rule started at the age of 17, when he was one of 2,000 "passive resisters" arrested in 1946 for defying laws that discriminated against Indian South Africans.

The ANC party was banned in 1960, and two years later Kathrada - often known as 'Kathy' - was placed under house arrest.

Soon afterwards, he went underground to continue the struggle as a member of the ANC's armed wing, Umkhonto we Sizwe (MK).

In July 1963, the police swooped on Liliesleaf Farm in Rivonia, outside Johannesburg, where Kathrada and other senior activists were meeting in secret.

At the famous Rivonia trial, eight of the accused were sentenced to life imprisonment with hard labour on Robben Island.

His fellow prisoners included Mandela, Walter Sisulu and Denis Goldberg.

"The nation has lost a titan, an outstanding leader and a great patriot," the ANC said.

"His life is a lesson in humility, tolerance, resilience and a steadfast commitment to principle."

The Nelson Mandela Foundation lauded him as "the embodiment of promise" during the apartheid years, saying Kathrada was "a comrade, associate and close friend of Nelson Mandela's through seven decades."

Released from prison in 1989, the softly-spoken Kathrada belonged to the golden generation of freedom fighters untainted by later corruption scandals.

In jail, he was a teacher for fellow prisoners and a strategic thinker who formed part of the ANC delegation in the negotiations that finally ended apartheid.

"He will be remembered as an unassuming freedom fighter, whose wisdom, tolerance, humility, steadfastness and humour earned him the love and respect of our people," said South Africa's Deputy President Cyril Ramaphosa.

"After the achievement of democracy and the adoption of a new democratic constitution... 'Kathy' was determined to continue to serve the people."

Kathrada chose not to pursue a political career, but remained an activist and he was critical of the current ANC government under President Jacob Zuma.

Fellow Robben Island prisoner Laloo "Isu" Chiba said Kathrada was a figurehead to all anti-apartheid fighters.

"He has been my strength in prison, my guide in political life and my pillar of strength in the most difficult moments of my life," Chiba said in a statement.

FW de Klerk, the white-minority president who ended the ban on the ANC, said that Kathrada was "a brave warrior for democracy, justice and non-racialism (who) leaves an indelible mark on generations of South Africans."

He was survived by his wife, Barbara Hogan, also an ANC stalwart.

Kathrada will have an Islamic burial on Wednesday, the foundation said, while the government ordered national flags to be flown at half-mast.