Anti-apartheid activist Ahmed Kathrada, who spent 26 years in jail – many of them alongside Nelson Mandela – for acts of sabotage against South Africa's white minority government, died in Johannesburg on Tuesday morning at the age of 87.

He had been admitted to hospital with blood clotting in his brain earlier this month.

Kathrada was born on August 21, 1929, to Indian immigrant parents in a small town in northwestern South Africa .

He was among those tried and jailed alongside Mandela in the Rivonia trial in 1964, which drew worldwide attention and highlighted the brutal legal system under the apartheid regime.

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Kathrada was sentenced to life imprisonment in 1964 and spent 26 years and three months in prison, 18 of which were on Robben Island.

After the end of apartheid, he served from 1994 and 1999 as parliamentary counsellor to President Mandela in the first African National Congress (ANC) government.

Al Jazeera's Tania Page, reporting from Johannesburg, said that it was a sad day in South Africa – where Kathrada was affectionately known as "Uncle Kathy" – as tributes poured in about his widely perceived kindness, humility, and honesty.

He had been a major part of many South African's memories over decades of anti-apartheid struggle, Page said.

"I think his passing is sort of signalling to South Africans, yet again, the ending of an era, of these great giants of apartheid [resistance] as they pass on," she added.

Kathrada gave an emotional speech at Mandela's funeral, in which he said he had lost a brother.

Kathrada was, until recently, still active in public life. He formed his own foundation and advocated strongly for human rights causes such as youth development, anti-racism, and freedom of speech.

Last year, he joined a movement of veteran figures who were critical of the governing ANC and its current crop of leaders - particularly President Jacob Zuma , who has been mired in mounting allegations of corruption . Kathrada penned an open letter to the president and called on him to step down.

"Right to the very end he kept himself relevant, he was a newsmaker, he was honest and true to his values and his beliefs," Page said. "And that's why so many millions of South Africans will be very sad at his passing today."

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"This is a great loss to the ANC, the broader liberation movement and South Africa as a whole," Neeshan Balton, head of the Ahmed Kathrada Foundation, said in a statement.

"'Kathy' was an inspiration to millions in different parts of the world."



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



In July 1963, the police swooped on Liliesleaf Farm in Rivonia, a Johannesburg suburb where Kathrada and other senior activists had been 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.