Johannesburg, South Africa (CNN) Former US President Barack Obama mounted a passionate defense of democracy and warned against the rise of "strongman politics," in a speech in South Africa a day after his successor, Donald Trump, was heavily criticized for a humiliating news conference with Vladimir Putin.

In an address Tuesday in honor of the late Nelson Mandela ahead of the 100th anniversary of his birth, Obama criticized populist movements toward authoritarianism around the world and ridiculed the "utter loss of shame among political leaders" who lie.

Obama has made an art of criticizing the current President's values without explicitly naming Trump , peppering his speech Tuesday with warnings against some of Trump's key policies, including protectionism, climate change denial and closed borders.

"The politics of fear and resentment and retrenchment began to appear. And that kind of politics is now on the move. It's on the move at a pace that would have seemed unimaginable just a few years ago," he told the crowd of around 15,000 people in Johannesburg.

"I am not being alarmist, I'm simply stating the facts. Look around — strongman politics are ascendant, suddenly, whereby elections and some pretense of democracy are maintained, the form of it, where those in powers seek to undermine every institution or norm that gives democracy meaning."

Former US President Barack Obama, right, with South African President Cyril Ramaphosa standing behind the Soweto Gospel Choir.

'People just make stuff up'

Obama's remarks come a day after Trump's news conference in Helsinki , Finland, in which the US leader sided with Putin over his own country's intelligence agencies on whether Russia interfered in the 2016 US election.

Trump had been expected by some observers to confront Putin over the issue after the US Department of Justice indicted 12 Russians, accused of hacking the Democrat's emails and computer networks to target Hillary Clinton's presidential campaign.

The President flip-flopped on his stance on the matter, having said last year he believed Russia was behind the interference, but back-pedaling Monday to claim he believed Putin's account that Russia was not involved.

Obama mocked the way politicians lie and reminded his audience of the importance of facts.

"You have to believe in facts. Without facts there's no basis for cooperation. If I say this is a podium and you say this is an elephant, it's going to be hard for us to cooperate," he said.

"I can't find common ground if somebody says that climate change just isn't happening, when almost all the world's scientists tell us it is. I don't know where to start talking to you about this. If you say it's an elaborate hoax, where do we start?"

Attendees of Obama's speech at the Wanderers Cricket Stadium in Johannesburg.

He added that politics today often rejected the concept of objective truth.

"People just make stuff up. They just make stuff up. We see it in the growth of state sponsored propaganda. We see it in internet fabrications. We see it in the blurring of lines between news and entertainment. We see the utter loss of shame among political leaders where they're caught in a lie and they just double down and they lie some more. It used to be that if you caught them lying, they'd be like, oh man -- now they just keep on lying," he said, to laughter in the crowd.

'Head-spinning and disturbing headlines'

Obama had opened his speech reflecting on the recent chaos of the world that gave him the opportunity to seek perspective.

"But in the strange and uncertain times that we are in -- and they are strange, and they are uncertain, with each day's news cycles bringing more head-spinning and disturbing headlines -- I thought maybe it would be useful to step back for a moment and try to get some perspective, so I hope you'll indulge me," he said.

Barack Obama, right, and South African President Cyril Ramaphosa.

He warned that the press was under attack, that censorship and state control of media is on the rise and that social media was being used to promote hate, propaganda and conspiracy theories.

"So, on Madiba's 100 birthday, we now stand at a crossroads," he said, using a clan name of affection for Mandela.

He said that there was a choice between two visions of humanity's future that the world must choose between.

"How should we respond? Should we see that wave of hope that we felt with Madiba's release from prison? From the Berlin Wall coming down? Should we see that hope that we had as naïve and misguided?" he asked.

Obama speaking in front of an image of Nelson Mandela.

"Let me tell you what I believe. I believe in Nelson Mandela's vision, I believe in a vision shared by Gandhi and King, and Abraham Lincoln, I believe in a vision of equality and justice and freedom and multi-racial democracy built on the premise that all people are created equal and are endowed by our creator with certain inalienable rights."

Obama's speech, the 16th annual Nelson Mandela Lecture, is one of his highest-profile appearances and his first return to Africa since he left office in 2017.

His lecture, titled "Renewing the Mandela legacy and promoting active citizenship in a changing world," tracked the transformation of the world, particularly in terms of race relations and human rights, over the past 100 years.

"It is a plain fact that racial discrimination still exists in both the United States and South Africa," he said.

Obama's speech followed remarks by South African President Cyril Ramaphosa, and Mandela's widow, Graça Machel, formerly a freedom fighter and minister in Mozambique's government.

Machel drew several parallels between Mandela and Obama, portraying them both as modest men as "symbols of victory over adversity."

Nelaon Mandela's widow, Graça Machel, center left, greeting Barack Obama on Tuesday.

"From the humblest of beginnings, they are representatives of the masses and reached to the pinnacle of power and influence. But in doing so they were able to elevate the rights and ambitions of the disenfranchised and the weak. Of young and old, of both men and women, of black and white," she said of both leaders.

Ramaphosa said South Africans celebrated Obama because he shared similar leadership qualities as Mandela and had the same ability to inspire hope and action.

Imprisoned for nearly three decades for his fight against state-sanctioned racial segregation, he was freed in 1990 and quickly set about working to unite the nation through forgiveness and reconciliation, becoming South Africa's first black president. Obama, who became the US' first black president in 2009, has referred to Mandela as a mentor.