JOHANNESBURG — U.S. President Barack Obama met the family of South Africa's ailing anti-apartheid hero Nelson Mandela on Saturday and he praised the critically ill, retired statesman as one of history's greatest figures.

The faltering health of Mandela, 94, a figure admired globally as a symbol of struggle against injustice and racism, is dominating Obama's two-day visit to South Africa. First Lady Michelle Obama and the Obama daughters did not attend the meeting.

Police fired stun grenades on Saturday to disperse several hundred protesters who had gathered outside the Soweto campus of the University of Johannesburg, where Obama was due to address a town hall meeting with students.

Obama's meeting with Mandela's family took place at the Nelson Mandela Centre of Memory in Johannesburg.

Obama told reporters afterwards he also spoke by telephone with Mandela's wife Graca Machel, who remained by her husband's side in the hospital in Pretoria.

"I expressed my hope that Madiba draws peace and comfort from the time that he is spending with loved ones, and also expressed my heartfelt support for the entire family as they work through this difficult time," he said, using the clan name Madiba by which Mandela is affectionately known.

Obama earlier had talks with South African President Jacob Zuma and the two held a joint news conference in which Zuma said Mandela remained in a "critical but stable condition."

But Mandela's deterioration in the last week to a critical condition forced the White House to rule out the possibility of Obama and his wife seeing the frail ex-statesman.

Zuma said Mandela had told him before his latest hospitalization that "when I go to sleep I will be very happy because I left South Africa going forward."