South African leader Jacob Zuma has reiterated that Nelson Mandela's condition is "critical but stable" but he said he hoped he would be out of hospital soon.

He made the comments after meeting with U.S. President Barack Obama, who says he views Mandela as a personal hero.

"The yearning for justice and dignity that transcends the boundaries of race, class, faith and country - that's what Nelson Mandela represents; that's what South Africa at its best can represent to the world; and that's what brings me back here," Obama said.

Obama is on a two-day state visit to South Africa to boost ties and redress concerns of US neglect on African partnerships.

He met the Mandela family members but not the man himself.

The president's trip has spurred protests in South Africa over drone attacks and the failure to shut Guantanamo Bay prison.

In Soweto, student demonstrators scuffled with police, angered over the University of Johannesburg's decision to award Obama an honorary doctorate.

Some said Obama did not deserve the honour, accusing him of human rights abuses.