UNITED NATIONS - The UN was set to mark Nelson Mandela International Day on Wednesday with a vow to continue the struggle for equality.

"Nelson Mandela was a towering global advocate for justice and equality," said United Nations Secretary-General Antonio Guterres on Tuesday in a video message ahead of the annual event, which was inaugurated by the UN General Assembly in 2009 in recognition of Mandela's global contribution to the culture of peace and freedom.

Mandela, who was born on July 18, 1918, and died on Dec 5, 2013, was the first democratically elected president of South Africa and the country's first black head of state.

"Rarely has one person in history done so much to stir people's dreams and move them to action," the UN chief said. "That struggle for equality, dignity and justice continues."

In December 2015, the General Assembly decided to extend the scope of Nelson Mandela International Day to also promote humane conditions of imprisonment and to encourage societies everywhere to treat prisoners as a continuous part of society by adopting the revised UN Standard Minimum Rules for the Treatment of Prisoners, known as the "Nelson Mandela Rules".

Also on Tuesday, in his highest profile speech since leaving office, former US president Barack Obama used a tribute to Mandela to warn that the world had plunged into "strange and uncertain times", in what is likely to be seen as a veiled attack on his successor Donald Trump.

Obama, who left the White House last year, has often credited Mandela for being one of the great inspirations in his life.

Obama made no direct reference to Trump 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".

He also blasted climate-change denial, race-based migration policies, unbridled capitalism and "strongman politics" - stances often cited as the hallmarks of Trump's presidency.

Obama spoke to a crowd of more than 10,000 people at a cricket stadium in Johannesburg in the centerpiece event.

"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.

On migration, he appeared to take a sharp jab at Trump, saying "it is not wrong to insist that national borders matter... but that can't be an excuse for immigration policies based on race or ethnicity or religion".

Xinhua - AFP