Anti-apartheid activist Winnie Madikizela-Mandela died in a Johannesburg hospital on Monday, AP reported. The former wife of South Africa’s first black President Nelson Mandela was 81.

Madikizela-Mandela was married to Nelson Mandela from 1958 to 1996. The couple led the struggle against apartheid in South Africa for nearly three decades. They spent 27 years of their married life away from each other, as Nelson Mandela was sent to prison in 1962 and was released in early 1990. Mandela died in 2013.

Madikizela-Mandela “succumbed peacefully in the early hours of Monday afternoon surrounded by her family and loved ones” following a long illness, a family spokesperson said. “She kept the memory of her imprisoned husband Nelson Mandela alive during his years on Robben Island and helped give the struggle for justice in South Africa one its most recognisable faces.”

Nobel laureate Desmond Tutu called her a “defining symbol of the struggle against apartheid”, BBC reported. “Her courageous defiance was deeply inspirational to me, and to generations of activists,” he said.

Madikizela-Mandela’s activism had become controversial in the later years when she was convicted in various cases for fraud, kidnapping and assault.