South African anti-apartheid activist Ahmed Kathrada died at the age of 87, his foundation said in a tweet on Tuesday. Known as “Uncle Kathy”, Kathrada had been sentenced to life imprisonment along with Nelson Mandela in 1964. He had spent 26 years in prison under an apartheid government, Reuters reported. Kathrada died of complications following a brain surgery.

“This is great loss to the African National Congress, the broader liberation movement and South Africa as a whole. Internationally, he was staunch in his support for the Palestinian struggle,” a statement by the Kathrada Foundation said. Kathrada was a staunch critic of President Jacob Zuma, who said the activist would be honoured with a state funeral and that flags at government offices would fly at half-mast until then.

The ANC veteran was born to immigrant India parents in Schweizer-Reneke, South Africa in 1929. He joined politics when he was 12, by distributing leaflets for the Young Communist League of South Africa, The Telegraph reported. His activist career spanned 57 years.

Nobel Peace Prize winner Archbishop Desmond Tutu said Kathrada had helped inspire the world’s confidence in South Africa’s long battle against white-minority rule, Reuters reported. “May he rejoice in many heavenly cups of hot chocolate with his old friends and comrades,” Tutu’s statement said.

In 2016, Kathrada had written an open letter to beleaguered president Jacob Zuma, asking him to resign after several scandals involving misappropriation of funds surfaced. “I am not a political analyst, but I am now driven to ask: 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?” he had written.