Johannesburg, South Africa (CNN) Internationally, Ahmed Kathrada didn't have the name recognition of his close friend and cellmate Nelson Mandela, but he was one of South Africa's best-loved and revered anti-apartheid activists -- and a moral compass for the nation.

So much so, many South Africans simply called him "Uncle Kathy."

Kathrada died Tuesday after a short illness following brain surgery, his foundation announced. He was 87.

"Comrade Kathy was a gentle, humane and humble soul. He was a determined revolutionary who gave his entire life to the liberation struggle of our country," said Derek Hanekom, the chairman of the Kathrada Foundation.

Born in rural northwest of the country to Indian immigrants, Kathrada became politically active at a young age, befriending Mandela and other emerging anti-apartheid leaders of the African National Congress (ANC) as the struggle against racist apartheid policies intensified.

After the apartheid government banned the ANC and other political parties, Kathrada went underground and joined the armed struggle. But in 1963, he was swept up in police dragnet that would nab the top leadership of the ANC.

After the infamous Rivonia trial, Kathrada and scores of other leaders, including Mandela, were convicted of treason and sentenced to life in prison with hard labor in the notorious Robben Island prison.

Kathrada spoke about how the sentence was a relief, because most of the Rivonia trialists expected to hang.

He would spend the next 26 years and three months in prison.

With other anti-apartheid leaders, Kathrada helped turn Robben Island into a university, attaining four university degrees in the process. The imprisoned activists began charting a way forward for a post-apartheid South Africa.

"He was a member of that generation of South African leadership which made a post-apartheid conceivable. His commitment to core values was unshakeable, inspiring a life-time of service and informing a rejection, right to the end, of the abuse of power in any form," said the Nelson Mandela foundation in a statement.

After leadership positions in Mandela's first post-apartheid government, Kathrada retreated behind the scenes, but emerged last year as one of the most senior ANC veterans calling for President Jacob Zuma to step down.

ANC: 'His life is a lesson in humility'

In an open letter, Kathrada criticized the President for a series of corruption scandals that have shaken South Africans and rocked investor confidence in the country.

"I know that if I were in the President's shoes, I would step down with immediate effect. I believe that is what would help the country to find its way out of a path that it never imagined it would be on, but one that it must move out of soon," Kathrada wrote.

On Tuesday , Zuma announced a special state funeral for Kathrada and flags will be flown half-mast throughout the country.

Kathrada died after a short illness following brain surgery.

"His life is a lesson in humility, tolerance, resilience and steadfast commitment to principle, even when taking a principled stance would place him at odds with his comrades," said the ANC in a statement.

Kathrada's death is the latest passing of veteran struggle activists that have been increasingly critical of the direction of the country.

Denis Goldberg, a close friend of Kathrada, and fellow Rivonia prisoner, called into a popular morning radio show to make an appeal.

"We fought apartheid because the resources and the valuable minerals and the lives of our people were stolen by apartheid and now we are stealing the very valuable things that our very constitution is supposed to defend. Of course we needed Kathy," he said.