Zindzi Mandela, the youngest daughter of South Africa's first black president Nelson Mandela and anti-apartheid activist Winnie Madikizela-Mandela, has died at the age of 59.

She died in a Johannesburg hospital in the early hours of Monday morning, public broadcaster SABC said.

She was serving as the South African ambassador to Denmark at the time of her death.

The cause of her death was not immediately revealed.

Zindzi Mandela was Nelson Mandela's sixth child and his second with Winnie Madikizela-Mandela, his second wife.

President Cyril Ramaphosa in a statement said Zindzi Mandela had "during our years of struggle brought home the inhumanity of the apartheid system and the unshakeable resolve of our fight for freedom".Skip Twitter post by @CyrilRamaphosa

Ms Mandela had been posted to Denmark in 2015 and had been designated to become South Africa's head of mission in Monrovia, Liberia.

Minister of International Relations Naledi Pandor said: "Zindzi will not only be remembered as a daughter of our struggle heroes, but as a struggle heroine in her own right."

Ms Mandela grew up at the height of the anti-apartheid struggle. With her father imprisoned on Robben Island, she endured years of harassment and intimidation by the apartheid regime, along with her sister Zenani, and her mother Winnie, says the BBC's Vumani Mkhize.

Zindzi Mandela was the family member who read out Nelson Mandela's rejection of then-president PW Botha's offer for conditional release from prison at a public meeting in February 1985.

Most recently, she was known for her vocal support for radical land reform in South Africa, our correspondent says.

Only two of Nelson Mandela's six children are still alive: Zenani Dlamini, Zindzi's sister; and Pumla Makaziwe Mandela, a daughter from his first marriage, to Evelyn Mase.