South Africa's first black president Nelson Mandela invited one of his former jailers to his home for a special dinner to help mark the 20th anniversary of his release from prison.

The Nobel Peace Prize winner spent 27 years in prison under white-minority apartheid rule, mainly on the notorious Robben Island near Cape Town. His release on February 11, 1990, set South Africa on the path to democracy.

Christo Brand was a Robben Island warder, but the two developed a friendship that Mandela said in his memoirs had "reinforced my belief in the essential humanity of even those who had kept me behind bars."

Brand was among a small group invited to Mandela's home late Thursday for a dinner with his ex-wife Winnie Madikizela-Mandela, his daughter Zindzi, and leading anti-apartheid activists who helped Mandela on the day when he walked free.

At the party Brand asked Mandela, a boxer in his youth, if he still exercised, according to the Sapa news agency.

"It's not easy, but I do it every now and then. I do feel like I am getting old. Time is flying. I'm not really worried," 91-year-old Mandela said.

Zindzi Mandela filmed the event for a documentary called "Conversations About That Day", which she plans to screen for the anniversary next week.

Cyril Ramaphosa, who played a key role in the negotiations that ended apartheid, toasted Mandela with the affectionate nickname "Tata", or father.

"You are still an inspiration," Ramaphosa said, according to Sapa. "We are forever indebted to you, for the leadership and inspiration you provided. We are happy you are a free man, because as you became free, you made us free. Thank you Tata."

South Africa's last apartheid president FW de Klerk announced that Mandela would be freed on February 2, 1990, as he unbanned the African National Congress (ANC) in a major step toward dismantling minority rule.

At the time, South Africa was an international pariah, under crippling international trade sanctions and engulfed in political violence.

De Klerk, who shared the Nobel prize with Mandela, said in a speech Tuesday that his decision to free Mandela had "prevented a catastrophe".

"We would, no doubt, have been able to maintain control for many years but under increasingly grim and unacceptable circumstances," said De Klerk.

"Our young men would have spent half their time in military service; many more white South Africans would have left the country; and there would have been pervasive white poverty and unemployment among those who remained.

"Worse still, the prospects for a satisfactory negotiated settlement would have diminished with each successive cycle of revolution and repression."

Mandela's insistance on reconciliation is widely credited with helping pull South African from the brink of civil war. He won the first all-race elections in 1994 and then served a single term as president.