Former South African President Nelson Mandela is showing a "great improvement" in his health compared to a few days ago, his ex-wife Winnie Madikizela-Mandela has said.

She said: "I'm not a doctor, but I can say that from what he was a few days ago there is great improvement."

Ms Madikizela-Mandela was speaking to reporters outside Mr Mandela's former home in the Johannesburg township of Soweto.

South Africans have been holding prayer vigils for the 94-year-old outside his former home.

President Jacob Zuma cancelled a scheduled trip to neighbouring Mozambique on Thursday because of the gravity of Mr Mandela's condition.

An official update said Mr Mandela remained critical but was "stable".

This is Mr Mandela's fourth hospitalisation in six months.

US President Barack Obama has arrived in South Africa on the second leg of a three-nation tour of Africa.

During his weekend trip to Johannesburg, Pretoria and Cape Town, Mr Obama is scheduled on Sunday to visit Robben Island, where Mr Mandela passed 18 of the 27 years he spent in apartheid prisons.

White House officials have said they will defer to the Mandela family on whether a visit to the hospital to see Mr Mandela would be appropriate.

Mr Obama has said he was not seeking a "photo op" with the ailing statesman.

Yesterday, Mr Obama said his thoughts and prayers were with the Mandela family and South Africa's 53 million people.

Speaking in Senegal, Mr Obama said the former South African president was a "personal hero".

"Even if he passes on, his legacy will linger on," he said.

Mandela grave dispute before court

A dispute between factions of Mr Mandela's family over where the family grave should be has gone to court.

His eldest daughter and more than a dozen other relatives sought an injunction against Mandela's grandson, Mandla.

State broadcaster SABC said a court in Mthatha had ordered Mandla to return the remains of three of Mandela's children from the village of Mvezo, where the anti-apartheid icon was born and where Mandla is now an influential tribal chief, to Qunu, the village 20km away where Mandela spent most of his childhood.

The three bodies were taken from the Mandela family cemetery in Qunu in the Eastern Cape two years ago and reburied in Mvezo, where Mandla, 39, has built a memorial centre that many have interpreted as an attempt to ensure Mandela is buried there.

Local media have reported that the exhumations took place at the behest of Mandla - officially the clan patriarch after the death of his father, Makgatho, in 2005 - and without the consent of other family members including Mandela's eldest daughter Makaziwe, who wants her father buried in Qunu.

The row between the Makaziwe-led Qunu faction and Mandla, an African National Congress member of parliament, has been brewing for months.

Lawyer Wesley Hayes, representing Makaziwe and 15 other relatives, confirmed to Reuters that papers had been filed in the Mthatha regional court against Mandla but refused to disclose details "because of the sensitivity of the case".

The court was not available to give details of its ruling.

The three Mandela children buried in Mvezo are an infant daughter who died in 1948, a son, Thembi, who died in a car crash in 1969, and Makgatho, who died of an AIDS-related illness in 2005.

In all, Mandela fathered six children from his three marriages.