South African Anti-Apartheid Leader Kathrada DiesSouth African anti-apartheid activist Ahmed Kathrada, who spent 18 years imprisoned alongside former President Nelson Mandela for their opposition to the racist regime, has died, his charitable foundation said Tuesday. Neeshan Balton, executive director of the Ahmed Kathrada Foundation, described his death in a Johannesburg hospital as "a great loss to the [ruling African National Congress party], the broader liberation movement and South Africa as a whole." The 87-year-old dedicated most of his life to fighting for equality in his native South Africa. He died early Tuesday after surgery for a blood clot in his brain.\n\nAs a boy of 12, he joined the nation’s Young Communist League, which opposed the racist apartheid regime. At 17, he was arrested for his resistance to laws that discriminated against South Africans of Indian origin. In 1964, he and six others stood in the dock alongside anti-apartheid icons Nelson Mandela and Walter Sisulu as a judge handed the group a life sentence for their efforts to disrupt white minority rule. Kathrada would spend the next 26 years in prison, 18 of them alongside Mandela, who became South Africa’s first black president in 1994 and was widely regarded as the father of the nation. At Mandela’s 2013 funeral, Kathrada tearfully remembered their relationship.  “When Walter died, I lost a father,” he said during the service in Qunu, Mandela’s rural hometown. “And now, I have lost a brother. My life is in a void, and I do not know who to turn to.” Kathrada’s death also feels like a family affair in South Africa. ''The struggle denied Ahmed Kathrada the opportunity to have children of his own,” Archbishop Emeritus Desmond Tutu said in a statement. "He was first imprisoned at the age of 17. But many South Africans looked up to him as a favorite grandparent." Shortly after his death was announced Tuesday, President Jacob Zuma called for an official funeral and instructed flags to fly at half-staff across the nation for the man he called “Uncle Kathy.” Sello Hatang, head of the Nelson Mandela Foundation, said Kathrada was more than just a friend to Mandela, who Hatang referred to by his clan name, Madiba. “Madiba said he surrounded himself with friends who could tell him when he was going wrong. And he considered Mr. K one of those friends, who could tell him as he was going wrong.” Mandela kept Kathrada close during his presidency, making him a top political advisor. In his later years, Kathrada started a charitable foundation.  “Comrade Kathy was a gentle, humane and humble soul,” said Kathrada Foundation Chairperson and tourism minister Derek Hanekom. “He was a determined revolutionary who gave his entire life to the liberation struggle in our country,” he added. In later years, he continued to be a voice of moral authority, and he directed some of his ire at Zuma, who has weathered a number of corruption scandals. Last year in an open letter, he called on Zuma to resign, saying, "Dear Comrade President, don’t you think your continued stay as president will only serve to deepen the crisis of confidence in the government of the country?" Hatang also urged South Africans to celebrate Kathrada’s life and to try to live up to his principles. “In his honor, may we continue to build a world that is more equal, not just in South Africa but globally,” he said. "That the vulnerable in our society are then included more and they are taken care of.” Kathrada will be buried Wednesday in Johannesburg in accordance with Muslim rites.