The most important part of Nelson Mandela's legacy is that the world's common humanity matters more than its differences, former United States president Bill Clinton said on Thursday.

"Our differences are wonderful, they make life more interesting, but our common humanity matters more," said Clinton at the Nelson Mandela Foundation offices in Johannesburg.

"It is the most important part of his legacy because it's at the heart of every single problem that our children and grandchildren will face."

Clinton was delivering a lecture at the opening of an exhibition commemorating Mandela and former African National Congress (ANC) president Chief Albert Luthuli.

Mandela, who celebrated his 89th birthday and ninth wedding anniversary on Wednesday, attended the event but did not make a speech.

Clinton said "divorce was not an option" in today's interdependent world, which faced problems of security, sustainability and inequality.

It was important for people across the world to remember and live by the practical implications of Mandela's legacy and not only its spirit, he said.

The 60-year-old statesman, who is involved in humanitarian work through the William J Clinton Foundation, said he tried to come to South Africa annually around the time of Mandela's birthday.

"I want to see my friend and because he's been the inspiration of much of what I've done.

"Now in the grace and beauty of his later years, he [Mandela] doesn't even have to say anything for us to know that you look better, you feel better, and you live better if you think our common humanity is more important than our ... differences," Clinton said.

"That is the legacy that the rest of us must keep alive for him."

The Luthuli and Mandela exhibition, entitled Making Peace, explores the relationship between the former ANC presidents through pictures and text.

Both leaders are Nobel Peace Prize laureates and Luthuli, who died 40 years ago, was the first African and second black person to be awarded the prize.

Earlier, Luthuli's daughter, Dr Albertina Luthuli, said reaching out to people had come naturally to her father.

"His charisma cut across the strong racial barriers of those days."

Former Robben Island prisoner and fellow treason trial defendant Ahmed Kathrada said Luthuli's chief legacy had been the principles of a non-racial, non-sexist, democratic South Africa.

"While the pariah apartheid regime chose to regard and punish him as an enemy, the democratic world responded by conferring upon him the Nobel Peace Prize -- the very first on the African continent to be so honoured."

Clinton said the award had come when he was a young schoolboy in the American south, which was grappling with civil-rights issues.

He told Luthuli's daughter that her father had an impact "far beyond the borders of this country" almost 50 years ago.

A question on whether he was "ready to be a spouse" to wife and presidential hopeful Senator Hillary Clinton raised smiles from the audience, including Mandela.

Clinton said Hillary had helped him with every race that he had run between 1974 and 2000.

He had helped her since 2000, the year she was elected to the US Senate.

"I've still got 20 [years] to go, so I'm going to do whatever she asks."

Hospital project

Meanwhile, Clinton will on Friday launch a multimillion-dollar hospital project in impoverished Malawi, where healthcare is dogged by poor facilities and a brain drain.

"Former president Clinton will perform a ground-breaking ceremony for the construction of the modern hospital, which will cost $70-million," Local Government Minister George Chaponda said on Thursday.

The Clinton/Hunter Initiative for Development will bankroll the construction of the 80-bed hospital and 23 staff houses at Neno district, 120km south of the commercial capital, Blantyre, the minister said.

Clinton is also expected to hold private talks with President Bingu wa Mutharika at the hilltop Sanjika Palace in Blantyre during his one-day visit, Chaponda added.

Malawi, where 60% of the 12-million citizens live below the poverty line of less than a dollar a day, faces an acute lack of health workers with about 120 registered nurses migrating to Britain and the United States every year in search of better salaries. -- Sapa-AFP