By Barnaby Phillips BBC, Stellenbosch

The recent conference of the African National Congress was a chance for President Mbeki to outline his vision of South Africa's future, before the party faithful, and a national television audience.

But halfway through President Mbeki's speech there was an interruption.

He knows there is a very big difference between what he is saying and what the president is saying Aids activist Mark Haywood

Nelson Mandela entered the hall, and was given a rapturous reception.

President Mbeki, visibly irritated, had to wait for several minutes before the clamour died down and he could resume his speech.

It was a telling moment - a cruel reminder for President Mbeki that he does not have, and never will have Nelson Mandela's personal touch.

Ranjeni Munusamy, a political reporter for the Johannesburg Sunday Times, says President Mbeki is often stiff and awkward.

"There have been times when he has had to do public appearances on television in the presence of children and he has been so uncomfortable."

She says that Mr Mbeki's appearances are very businesslike and all about work.

"It has nothing to do with the romanticism and the sentiment that defined the Mandela presidency"

Aids

But the differences are not just of style, but also of substance.

Mbeki is at loggerheads with experts over Aids

The most critical issue facing South Africa is the Aids pandemic - about 4.5 million people are HIV positive.

But President Mbeki is strangely uncommitted - he doubts the link between HIV and Aids, and he questions the value of anti-retroviral drugs.

Nelson Mandela, on the other hand, has thrown himself into the fight against Aids.

During a highly symbolic visit by Nelson Mandela to meet Aids campaigners, who are openly critical of President Mbeki, Mr Mandela showed support, and voiced his concern.

"What worries everybody is the number of people who are dying - almost daily."

It is an unremarkable comment - but when President Mbeki is so reluctant to talk about Aids at all, it is significiant.

Mark Heywood, a leading Aids activist, welcomes Nelson Mandela's approach.

"He knows there is a very big difference between what he is saying and what the president is saying," he said.

Mr Mbeki is likely to lead South Africa until 2009

"It's almost as if Nelson Mandela wants to be known for getting the country to take the Aids issue seriously."

Private differences are unlikely to lead to public disputes between the two men.

Nelson Mandela is too loyal to the ANC, whilst President Mbeki is too aware of Mr Mandela's enduring populatity, to risk an open spat.

But that will not stop South Africans from gossiping and speculating on the tensions between President Mbeki, and former President Mandela.