Former US president Barack Obama yesterday used a tribute to Nelson Mandela to warn of "strange and uncertain times", a day after Donald Trump refused to challenge Vladimir Putin over interference in US elections.

Delivering a speech in Johannesburg marking 100 years since Nelson Mandela's birth, Obama made no direct reference to his successor but warned that the "politics of fear and resentment" were spreading across the world.

Obama criticised climate-change deniers, race-based migration policies, unbridled capitalism and "strongman politics" -- all issues likely to be seen as veiled attacks on Trump.

"Given the strange and uncertain times we are in, each day's news cycles brings more head-spinning and disturbing headlines, I thought maybe it would be useful to step back for a moment and get some perspective," Obama said at the start of his speech.

Obama spoke to a crowd of more than 10,000 people at a cricket stadium in Johannesburg in the centrepiece event of celebrations marking 100 years since Nelson Mandela's birth.

"It is in part because of the failures of governments and powerful elites... that we now see much of the world threatening to return to an older, more dangerous, more brutal way of doing business," Obama said.

Yesterday's speech came on the eve of "Mandela Day" -- his birthday, which is marked around the world every year on July 18.

Obama has made relatively few public appearances since leaving the White House in 2017, but he has often credited Mandela for being one of the great inspirations in his life.

Mandela, who died in 2013, remains a global icon for his long struggle against white-minority apartheid rule and for his message of peace and reconciliation after being freed following 27 years in prison.

The "Mandela 100" anniversary has triggered a bout of memories and tributes to the late anti-apartheid leader, as well as a debate over his legacy and South Africa's fate since he stepped down in 1999.

African President Cyril Ramaphosa and Mandela's widow Graca Machel were among the guests from Obama's speech.

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.