Winnie Madikizela-Mandela has described how her former husband and anti-apartheid hero Nelson Mandela drew his last breath.

Madikizela-Mandela, 77, was called to the deathbed of her ex-husband of 38 years at his home in Johannesburg's suburb of Houghton by worried relatives a few hours before he passed away.

"I phoned the doctor and he said 'No Mama, I think you'd better visit'. He had never used that word before. When he spoke like that then, I knew there was a very serious problem," she told ITV.

Madikizela-Mandela said she sat next to him for several hours, witnessing the slow decrease of heartbeat and blood pressure readings on Mandela's life-support machine.

"When I got there, the heartbeat was around 67 and the blood pressure was 55. I watched those figures going down and down so slowly. They felt eternal. They kept dropping and dropping," she said.

"The doctors were standing around him. They told me I should move close to him.

"I went close to him and I noticed he was breathing really slowly. I was holding him trying to feel his temperature and he felt cold. Then he drew his last breath and just rested. He was gone," Madikizela-Mandela said.

Mandela died aged 95 last week. Madikizela-Mandela was his second wife. The couple married in 1958 and split up within two years of Mandela's release from prison in 1990 and finally divorced in 1996.

Mandela's body is lying in state for the third and final day at the Union Buildings in Pretoria where he was sworn in as South Africa's first black president in 1994.

Madikizela-Mandela said it was "very painful" to see him lying on display in a coffin.

"In our African tradition we don't display the departed," she said. "It's very hard for the family to even share him even in his death after sharing with the whole world and our whole country while he was alive.

"He's still not really just ours, the family. He still belongs to the whole world and we have to share," she said.

Tens of thousands mourners queued to pay their last tribute to the iconic leader.

"There is no greater reward than the one we both had of a liberated South Africa," Madikizela-Mandela said.

Mandela's body is to be flown to Qunu, his ancestral home in the Eastern Cape Province, for burial over the weekend.