"Right now I am surprised by how much money I have, and I don't have half as much as these guys," Mr Obama tells the crowd.

"There is only so much you can eat, only so big a house you can have... I mean, it is enough.

"You don't have to take a vow of poverty just to say let me help out a few of these folks. I'll pay a little more in taxes, it is okay, I can afford it."

History shows how easily people can turn on those who look different or worship God in a different way, Mr Obama says.

More effort will have to made to fulfill Mr Mandela's vision for the future, he says.

"We are going to have to work harder and work smarter," Mr Obama adds.

Barack Obama says he "believes in Nelson Mandela's vision" for the world's future.

"I believe that a world governed by such principals is possible," he tells the crowd. "It can achieve more peace and more cooperation in pursuit of a common good...

"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."

In the US, challenges to globalisation first came from the political left, and then forcefully from the right, Mr Obama says.

The devastating effect of the 2008 financial crisis - and the reckless behavior of elites - made assurances given to people ring hollow, he adds.

"The politics of fear and resentment and retrenchment began to appeal. And that kind of politics is now on the move," Mr Obama says.

He adds that he not being alarmist, but he is just stating the facts.

During an era of strongmen politics only a pretence of democracy is being maintained, Mr Obama says.

"Strongman politics are ascendant suddenly... Those in power seek to undermine every institution or norm that gives democracy meaning," he adds.

Barack Obama points out that "in their business dealings, many titans of industry.. are increasingly detached from any nation state".

They "live lives more and more isolated from ordinary people", he adds.

As a result, their decisions to "shut down a factory" or move their money to a tax haven are "often down without malice", seen as simply a "rational response" to shareholders' demands.

The new economic order in the world has led to corruption on an "epic scale", Mr Obama says.

It has also given business people huge influence in politics, he adds.

It is a "plain fact" that racial discrimination still exists in the US and South Africa, Mr Obama says.

There are also "yawning disparities" in income and wealth, Mr Obama says.

"For many people, the more things change the more things stay the same," he adds.

In fact, he points out, the advances in the world have "meant that a few dozen individuals control the same amount of wealth as the poorest half of humanity".

Mr Obama says an entire generation has grown up in a world which has become freer and more tolerant.

"It should make us hopeful," he adds.

But world is threatening to return to an older, more dangerous and more brutal way of doing business, he says.

"We have to start by admitting whatever laws may have existed on the books... the previous structures of privilege and power and injustice and exploitation never completely went away, they were never fully dislodged," Mr Obama concludes, to cheers from the crowd.

A crowd of about 15,000 is listening to Mr Obama's speech

He says hope swept through the hearts of people when Mr Mandela was released from prison in 1990 - that "ancient hatreds" were crumbling.

Image source, Getty Images

"At the outset his struggle was particular to this place, to his homeland," Mr Obama tells the crowd.

"A fight to end apartheid.. but through his sacrifice and unwavering leadership and, perhaps most of all, through his moral example, Mandela and the movement he led would come to signify something larger."

It came to embody "the possibility of a moral transformation in the conduct of human affairs", Mr Obama adds.

"Mandela's light shone so brightly even from that narrow Robben Island cell that, in the late 70s, he could inspire a young college student on the other side of the world to reexamine his own priorities."

Barack Obama is looking back at the "remarkable transformation" which has taken place in the last 100 years.

"The average person [100 years ago] saw no possibility of advancing from the circumstances of their life," he said.

"Even in democaracies like the United States... racial segregation and systemic discrimination was the law in almost half the country and the norm in the other half.

"That was the world just 100 years ago."

A BBC reporter tweets:

We are living in strange and uncertain times, Mr Obama says.

Each day brings disturbing headlines, he adds.

The audience reacts - could this be a nod towards the man that followed him into the White House?

Mr Obama says he wants to correct something Mr Ramaphosa said.

"I am a very good dancer. I just want to be clear about that," he says.

Mr Obama has gone on to make a series of "confessions" to the audience.

"I was not exactly invited to be here. I was ordered in a very nice way to be here by Graca Machel," he tells the crowd.

"Confession number two: I forgot my geography and the fact that right now it is winter in South Africa.

"I didn't bring a coat, and this morning I had to send someone out to the mall because I am wearing long johns."

"I was born in Hawaii," he added by way of explanation.

Mr Obama has now taken the podium, following the end of Ramaphosa's speech.

"Thank you so much," Mr Obama says.

The crowd is on its feet, welcoming him.

Many people around the world dream of being like Nelson Mandela, Mr Ramaphosa says.

Mr Obama found ways of being like Mandela - he was a Nobel Preace Prize winner, for instance, Mr Ramaphosa says.

He continued: "Like Madiba, he was the first African-American president to lead his nation.

"Like Madiba, he is an inspiration to all those who are working and seeking to create a better world.

"Like Madiba, he has an abiding love and commit to empower young people."

But then Mr Ramaphosa changed his tone.

"Much as there are many similarities, there is one area where President Obama cannot match Madiba," he said.

"Unfortunately he cannot dance as well as Madiba can dance."

Mr Obama, luckily, looked like he could take a joke.

Image source, Reuters