"It is hard to eulogise any man ... How much harder to do so for a giant of history, who moved a nation towards justice," Obama said, while addressing hundreds and thousands of people gathered for Mandela's memorial service at the FNB stadium in Soweto, amid loud cheers.
"Born during World War I, far from the corridors of power, a boy raised herding cattle and tutored by elders of his Thembu tribe - Madiba (Mandela's clan name) would emerge as the last great liberator of the 20th century," Obama said.
"Like Gandhi, he would lead a resistance movement - a movement that at its start held little prospect of success. Like King, he would give potent voice to the claims of the oppressed, and the moral necessity of racial justice," Obama told the gathering with nearly 100 world leaders, including President Pranab Mukherjee, in attendance.
"Emerging from prison, without the force of arms, like Abraham Lincoln, he held his country together when it threatened to break apart," Obama said to applause from the over 80,000 crowd who braved the rain to attend the rare event.
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Earlier, South African President Jacob Zuma was booed by the crowd at the stadium when his images flashed up on the big screens.
In contrast, Obama received a huge ovation as he arrived an hour late for the emotional event, one of the largest such gatherings in generations.
"He has done it again," the UN Secretary General said, referring to leaders of different political ideologies gathered to pay homage to Mandela.
"Look around this stadium and this stage. We see leaders representing many points of view, and people from all walks of life. All here, united," Ban said.