Former U.S. President Barack Obama gave his highest-profile speech since leaving office on Tuesday in Johannesburg, South Africa at an event marking the 100th anniversary of anti-apartheid leader Nelson Mandela's birth.

Without ever directly mentioning his successor, President Donald Trump, Obama countered many of Trump's policies, rallying people to keep alive the ideas that Mandela worked for, including democracy, diversity and good education for all.

Obama opened by calling today's times "strange and uncertain," adding that "each day's news cycle is bringing more head-spinning and disturbing headlines." These days "we see much of the world threatening to return to a more dangerous, more brutal, way of doing business," he said.

He targeted politicians pushing "politics of fear, resentment, retrenchment," saying they are on the move "at a pace unimaginable just a few years ago."

He warned that countries that engage in xenophobia "eventually ... find themselves consumed by civil war," saying that people who put others down are "small-minded."

"I would have thought we had figured that out by now," Obama added, invoking the World Cup-winning French team and its diversity.

He also noted the "utter loss of shame among political leaders when they're caught in a lie and they just double down and lie some more," warning that the denial of facts — such as that of climate change — could be the undoing of democracy.

But Obama reminded the audience that "we've been through darker times. We've been through lower valleys," and he closed with a call to action: "I say if people can learn to hate, they can be taught to love." He added that "the struggle for basic justice is never truly finished."

He received a standing ovation.

South Africa's president introduced Obama as the rare person who dreamed about being like Nelson Mandela and achieving that dream, in front of an audience of 14,000.

President Cyril Ramaphosa, however, also poked fun at Obama, saying he cannot dance as well as Mandela could dance. Obama responded with a mock look of outrage and a smile.

This is Obama's first visit to Africa since leaving office in early 2017. He stopped earlier this week in Kenya, where he visited the rural birthplace of his late father.

Obama's speech highlighted how the Nobel Peace Prize winner, who was imprisoned for 27 years, kept up his campaign against what appeared to be insurmountable odds to end apartheid, South Africa's harsh system of white minority rule.

Mandela, who was released from prison in 1990 and became South Africa's first black president four years later, died in 2013, leaving a powerful legacy of reconciliation and diversity along with a resistance to inequality, economic and otherwise.

Obama has shied away from public comment on Trump, whose administration has reversed or attacked notable achievements of his predecessor. The U.S. under Trump has withdrawn from the 2015 Paris climate agreement and the Iran nuclear deal while trying to undercut the Affordable Care Act or "Obamacare."

Instead of commenting on politics, Obama's speech drew on broader themes and his admiration for Mandela, whom America's first black president saw as a mentor.

When Obama was a U.S. senator he had his picture taken with Mandela. After Obama became president he sent a copy of the photo to Mandela, who kept it in his office. Obama also made a point of visiting Mandela's prison cell and gave a moving eulogy at Mandela's memorial service in 2013, saying the South African leader's life had inspired him.