Nelson Mandela is being treated in a hospital near where Obama protesters are rallying.

South Africans are protesting Obama's visit by rallying in Pretoria, blocks from the hospital where anti-apartheid icon and Nobel Peace Prize winner Nelson Mandela lies critically ill.

Obama is on a three-nation tour of Africa. He will arrive in South Africa on Friday, but White House officials plan to defer to Mandela's family on the question of a visit between the first black president of the United States and the first black president of South Africa.

This is Nelson Mandela's fourth hospitalization in six months. Mandela, 94, is currently battling a lung infection and in critical condition. It is unknown if he will recover.

Meanwhile, gathering protesters nearby say "Mandela would be disappointed" by Obama. They waved signs calling him a "slave master" and asking, "Why no truth and reconciliation commission in the US?" One sign called on Obama to "stop the war on freedom of the press and whistle blowers."

Obama's foreign policy is "arrogant, selfish and oppressive," the students and union workers gathered say. They objected particularly to his support of drone strikes killing civilians and failure to close the Guantanamo Bay prison.

"We had expectations of America's first black president. Knowing Africa's history, we expected more," Khomotso Makola, a 19-year-old law student, told press.

"He has come as a disappointment, I think Mandela too would be disappointed and feel let down," Makola said.

Obama is making his first significant trip to Africa. He has said Mandela is "a personal hero".

"If and when he passes from this place, one thing I think we'll all know is that his legacy is one that will linger on throughout the ages," he told press.