Kathrada, a fellow inmate with Mandela at the Robben Island prison off Cape Town and an old friend, said he always considered the iconic leader to be his elder brother.
In an emotional eulogy at the funeral of the former South African president at his ancestral home of Qunu, Kathrada said Mandela "united the people of South Africa and the entire world on a scale never before witnessed in history."
85-year-old Kathrada, whose parents hailed from Surat in Gujarat, recalled the death of another top anti-apartheid leader Walter Sisulu 10 years ago, saying "When Walter died, I lost a father. And now, I have lost a brother. My life is in a void and I don't know who to turn to."
Listing the name of other late African National Congress (ANC) stalwarts, he said Mandela had now left to join many other "remarkable men and women, all of whom had played a critical role in the liberation of our country."
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"Ferocious struggles for democracy unfold daily before our very eyes and the numbers of political prisoners grow in step with rising intolerance. All of these people and prisoners throughout the world will continue to draw inspiration from the life and legacy of Mandela," he said.
With his voice quivering from emotion at times, Kathrada described the last moments of his meeting with the man with whom he spent 26 years in prison.
"It brought me to the verge of tears when my thoughts automatically flashed back to the picture of the man I grew up under. How I wished I'd never had to confront the reality of what I saw."
At the end, Kathrada said: "farewell my elder brother, my mentor, my leader. With all the energy and determination at our command, we pledge to join the people of South Africa and the world to perpetuate the ideals and values for which you have devoted your life.