Former US president Barack Obama has warned the world to resist the rise of “political strongmen” in what has been seen as a thinly-veiled swipe at his successor Donald Trump.

In his most high profile address since leaving office, Mr Obama said in a speech in Johannesburg to mark the 100th anniversary of the birth of Nelson Mandela on Wednesday morning (AEST) that the world is witnessing a rise of state-sponsored propaganda.

While he did not name Mr Trump, the speech was among the most pointed comments Mr Obama has made about politics since leaving office in January 2017.

“Just as people spoke about the triumph of democracy in the 90s, people now are talking about the triumph of tribalism and the strong man. But we need to resist that cynicism,” Mr Obama said.

Using language echoing that of Mr Trump’s critics, he spoke of an “utter loss of shame among political leaders”.

“Too much of politics today seems to reject the very concept of objective truth. People just make stuff up. We see it in the growth of state-sponsored propaganda, we see it in internet-driven fabrications, in the blurring of lines between news and entertainment,” Mr Obama said.

“We see the utter loss of shame among political leaders, where they are caught in a lie and they just double down and they lie some more,” he added.

Mr Obama said there were far-right parties in the West that have platforms of protectionism and closed borders but also a “barely hidden racial nationalism”.

Since leaving the White House, Mr Obama has largely avoided direct involvement in US politics and has refrained from criticising Mr Trump. Some Democrats say he should play a more direct role.

Mr Obama’s comments came shortly before Mr Trump dramatically backed down in the face of near universal condemnation of his summit with Vladimir Putin, saying he “misspoke” when he supported the Russian President’s denials of election interference.

On Tuesday morning (AEST), Mr Trump sided with Mr Putin over alleged Russian meddling in the 2016 US presidential election, casting doubt on the findings of his own US intelligence agencies and sparking a storm of criticism at home.

During Mr Obama’s speech, given in a stadium to thousands of politicians, businesspeople and students, the former president lauded Mr Mandela’s life of sacrifice and commitment to social justice.

Mr Mandela, a giant of the struggle against apartheid who served as South Africa’s president from 1994 to 1999, died in 2013 aged 95.

Mr Obama, who also visited Kenya this week, also praised South Africa’s current president, Cyril Ramaphosa, saying he had instilled “new hope” among South Africans.

-with AAP