Mandela lecture: Five things Barack Obama said
Barack Obama has used his first high-profile speech since stepping down as US president to take swipes at "strongman politics" and politicians' disregard for the facts.
His comments are seen as thinly veiled criticism of the current US administration's use of what has been described as "alternative facts".
Here are five key points from his Nelson Mandela lecture, made to the world's media and an audience of some 15,000 people in South Africa's main city, Johannesburg.
1. Facts are sacred
"You have to believe in facts," said Mr Obama, "without facts there is no basis for cooperation.
"If I say this is a podium and you say this is an elephant, it is going to be hard for us to cooperate."
Mr Obama said that he could find common ground with people who disagreed with the Paris accord on climate change - which Donald Trump wants to pull the US out of - if they had an argument based on fact.
But, Mr Obama added: "I can't find common ground if someone says climate change is not happening when almost all the world's scientists say it is. If you start saying it is an elaborate hoax, where do we start?"
Mr Trump has said he thinks climate change is not happening.
A moment later, Mr Obama told the audience: "It used to be if you caught them [politicians] lying, they said: 'Oh man'. Now they just keep on lying."
2. Immigration is a strength
Barack Obama said striving towards equality ensures a society can draw on the talents and the energy of all of its people.
"Just look at the French football team," he said to wild cheers, referring to France's World Cup win days before.
"Not all of those folks looked like Gauls to me, but they are French - they are French."
It is a "plain fact" that racial discrimination still exists in the US and South Africa, Mr Obama said.
'The ghost of US presidents past'
Analysis by BBC North America reporter Anthony Zurcher
Barack Obama offered what some of his supporters will see as not-so-veiled references to his successor in his speech honouring Nelson Mandela's legacy, defending democratic institutions and a free press, and condemning "strongman politics" and shameless leaders who "double down" when caught lying.
The former president also offered a commodity he always seems to have in ready supply - hope.
"Things may go backwards for a while, but ultimately, right makes might," Mr Obama said. "Not the other way around."
It's a riff on the Theodore Parker line he frequently quotes, about the arc of history being long but bending toward justice.
If Mr Obama had a message for the world - and particularly for Americans unsure about the course their nation is on - it's that the struggle is real, but the ending is a happy one.
There are probably more than a few on the left, however, who wish Mr Obama would give more than a few speeches and carefully worded statements.
With mid-term elections that will determine control of Congress just four months away, they want him to step away from the podium and fully join the fight.
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3. Business titans are 'isolated from ordinary people'
Barack Obama said the world's elite were out of touch with the lives of the poor.
"In their business dealings, many titans of industry... are increasingly detached from any nation state" and they "live lives more and more isolated from ordinary people", he said.
As a result, their decisions to "shut down a factory" were seen as simply a "rational response" to shareholders' demands.
4. Viva democracy!
Politicians using "politics of fear, resentment, retrenchment" were rising "at a pace unimaginable just a few years ago," Barack Obama warned.
Democracy is messy, he said, "but the efficiency of an autocrat is a false promise".
"It is time for us to stop paying all of our attention to the world's capitals... and focus on the world's grassroots. That is where democracy comes from," he added.
Warning against creeping populism and "strongman politics", he made the case for liberal democracy, saying that he believed it offered the better future for humanity.
"I believe in Nelson Mandela's vision" for the world's future, he said, "I believe that a world governed by such principles is possible".
"It can achieve more peace and more cooperation in pursuit of a common good," he added.
"I believe we have no choice but to move forward... I believe it is based on hard evidence. The fact that the world's most prosperous and successful societies happen to be those which have most closely approximated the liberal progressive ideal that we talk about."
Things may go backwards for a while, but - ultimately - right makes might," Mr Obama said. "Not the other way around."
5. Keep hope alive
"Keep believing. Keep marching. Keep building. Keep raising your voice. Every generation has the opportunity to remake the world," Mr Obama said, ending his speech on a positive note.
To loud cheers, he called on young people listening to him to get "fired up".
"We don't just need one leader... what we badly need is that collective spirit," he added.
"Mandela said young people are capable when aroused of bringing down the towers of oppression and raising the banners of freedom," he said. "Now is a good time to be aroused."
Barack Obama's speech was part of events to mark 100 years since the birth of former South Africa President Nelson Mandela, who died in 2013 aged 95.
Both men were the first black presidents of their countries.
Mr Obama has said he was "one of the countless millions who drew inspiration from Nelson Mandela's life".
Mandela led the fight against white minority rule in South Africa. He was imprisoned for 27 years before he became the country's first democratically elected president in 1994.
As a student, Mr Obama called the fight against apartheid "a struggle that touches each and every one of us", and encouraged his university to drop its investments in South Africa.
Since its beginning in 2003, global leaders have used the lecture to speak about issues affecting South Africa, the continent and the world.