Nelson Mandela's ex wife Winnie Madikizela-Mandela, 77, has described how she watched as the heartbeat and blood pressure readings on Mr Mandela's life-support machine slowly dropped off as he died.

She sat at his side for three and a half hours until he finally drew his last breath, she said in an interview with ITN.

"I received a call in the morning and I was told ... there were difficulties about Tata," she said, referring to Mr Mandela by his Xhosa name, which means "Father".

"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."

When she arrived at Mr Mandela's house, other relatives told her to go upstairs, to where doctors were gathered around him. "I asked 'is this it?'"she said. "Of course they kept quiet and they did not tell me.

"I sat there for three and a half hours, watching that machine. But the first shock I got was the fact they had switched off the dialysis machine and there was just a respirator registering the heartbeat and blood pressure. 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."

After Mr Mandela's readings dropped further still, she knew his very last moments were imminent, 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."

She added: "I realised all along as human beings I honestly could not find myself saying 'it is time' but I knew we had reached the end. You get this numb feeling. You don't react to that. I can't describe that kind of sorrow. Even though he was 95 and had done so much, there was so much that was still not done."

After the doctors had prepared Mr Mandela's body, President Jacob Zuma arrived, as did a group of soldiers to pick the body. "They saluted and marched upstairs and they came down so ceremoniously," said Mrs Madikizela-Mandela. "The whole thing was so official. It struck me then, he was gone and that was the last journey for him."

Asked how it was to see him lying in state, she replied. "Very painful. In our African tradition we don't display the departed. 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."

Mrs Madikizela-Mandela split from her husband within two years of his release from prison in 1990. South Africa's Truth and Reconciliation Commission later heard from several witnesses that she had headed a Soweto-based vigilante gang that in 1988, carried out the abduction and murder of alleged police informants and traitors to her husband's cause.

Mrs Madikizela-Mandela, who is a prominent ANC MP, has denied the accusations.