Zindzi Mandela, the daughter of South African anti-apartheid leaders Nelson Mandela and Winnie Madikizela-Mandela, has died aged 59.

State television South African Broadcasting Corporation has reported that Mandela died at a Johannesburg hospital early Monday morning. The cause of her death has not been announced.

She had been South Africa’s ambassador to Denmark since 2015.

The Mandelas’ daughter came to international prominence in 1985, when the white minority government offered to release Nelson Mandela from prison if he denounced violence perpetrated by his movement, the Africa National Congress, against apartheid, the brutal system of racial discrimination enforced in South Africa at that time.

Zindzi Mandela read his letter rejecting the offer at a packed public meeting that was broadcast around the world.

Last year Mandela stirred controversy by calling for the return of the white-owned land to South Africa’s dispossessed Black majority.

“Dear Apartheid Apologists, your time is over. You will not rule again. We do not fear you. Finally #TheLandIsOurs,” she tweeted in June last year.

South Africa’s foreign affairs minister Naledi Pandor has expressed shock at Mandela’s death, describing her as a heroine.

“Zindzi will not only be remembered as a daughter of our struggle heroes, Tata Nelson and Mama Winnie Mandela but as a struggle heroine in her own right. She served South Africa well,” said Pandor.

She is survived by her husband and four children.