He said Mandela was one of the most influential personalities of the century who taught the world true meaning of forgiveness and reconciliation and steered South Africans on to the path of building a truly rainbow nation.
UN Secretary General, who was the first foreign dignitary to pay homage, said termed Mandela as a "greatest teacher" and said "He was one of our greatest teacher who taught by example."
Obama, while paying tributes to his fight against anti-apartheid, said "we know that like South Africa, the United States had to overcome centuries of racial subjugation.
"As was true here, it took the sacrifice of countless people - known and unknown - to see the dawn of a new day. Michelle and I are the beneficiaries of that struggle," he said.
Obama regretted that around the world today men and women were still imprisoned for their political beliefs; persecuted for what they look like, or how they worship, or who they love.
"There are too many of us who happily embrace Madiba's legacy of racial reconciliation, but passionately resist even modest reforms that would challenge chronic poverty and growing inequality.
"There are too many leaders who claim solidarity with Madiba's struggle for freedom, but do not tolerate dissent from their own people," he said in remarks interpreted as a criticism of Cuban President Raul Castro with whom he shook hands.
Obama, who arrived as the memorial service was on, greeted Mukherjee warmly and shook hands with him.
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