S Africa pays tribute to Mandela on 100yrs of his birth

South Africa on Wednesday marked 100 years since Nelson Mandela's birth, with Barack Obama hosting a youth leadership project as archbishop Desmond Tutu led emotional tributes to the late anti-apartheid icon.

Mandela's birthday on 18 July is celebrated annually around the world with charitable works and the Nelson Mandela Foundation called for people to "take action and inspire change" in his name on the centenary year.

In Johannesburg, Obama told 200 young people attending a leadership course he has set up that Mandela had been a key inspiration in his life.

"Most people around the world think of Mandela as an older man, with hair like mine," he joked.

"What people of course don't recall is that he started as a very young man -- your age -- trying to liberate his country. He then inspired me."

Obama on Tuesday delivered the centrepiece address of the celebrations to a crowd of 15,000 at a cricket stadium, recalling the "wave of hope that washed through hearts all around the world" when Mandela was released from jail in 1990.

"Through his sacrifice and unwavering leadership and, perhaps most of all, through his moral example, Mandela... came to embody the universal aspirations of dispossessed people," Obama said.

Retired archbishop Desmond Tutu, 86, recalled fond memories of his fellow campaigner against white-minority rule and fellow Nobel Peace Prize winner.

"The most extraordinary thing about Nelson Mandela was his ordinariness. He was just a particularly fine example of humanity," Tutu said in a video message.

"The principles on which he led his life are universal principles of love, fairness and respect of others built on understanding of the vulnerabilities that we all share."

Tutu recalled Mandela's willingness to apologise when wrong, and laughed as he remembered that the two used to tease each other over Mandela's famous colourful shirts and Tutu's purple clerical robes.

- 'He led us from oppression' -

South African president Cyril Ramaphosa, a protege of Mandela who took office this year, said he would mark "Mandela 100" by donating half his salary to charity and called for others to do the same.

"He led us from the wilderness of conflict and oppression into the land of promise, of freedom, democracy and equality," Ramaphosa said.

Ramaphosa spent Mandela Day in Mvezo in Eastern Cape province, Mandela's birthplace, at celebrations including the opening of a clinic, a tree planting ceremony and distributions of blankets to elderly citizens.

Machel Graca, Mandela's widow, led a short walk to promote Mandela's legacy, while the centenary was marked with the release of a new book of his prison letters and a commemorative bank note.

Among a series of exhibitions, music festivals and sports events worldwide, a memorial stone was also due to be unveiled at a special service in London's Westminster Abbey.

Delivering the Nelson Mandela Annual Lecture on Tuesday, Obama warned that the world had plunged into "strange and uncertain times", in a speech containing a series of apparent veiled attacks on US president Donald Trump.

Obama made no direct reference to his successor but warned that "politics of fear and resentment" were spreading, driven by leaders who scorned facts and told lies with an "utter loss of shame".

The former US president met Mandela only briefly in 2005 but gave a eulogy at his funeral, hailing him as "the last great liberator of the 20th century".

Mandela was imprisoned under apartheid rule in 1962 and only freed in 1990, when he went on to lead the African National Congress party to victory in the first multi-race elections in 1994.

He served one term as president before stepping down in 1999. He died in 2013.

The year commemorating Mandela's birth will finish with a concert in Johannesburg in December starring Beyonce, Jay-Z, Ed Sheeran, Pharrell Williams and Cassper Nyovest.