Mandela, South Africa's first elected black president and a colossus of 20th century politics, died at his home in Houghton, a suburb of Johannesburg, following a lengthy illness. He was 95.
"Our beloved Nelson Rolihlahla Mandela, the founding President of our democratic nation, has departed," South African President Jacob Zuma said in a televised address to the nation.
"Our nation has lost its greatest son. Our people have lost a father," he said.
Describing himself as one of the countless millions who drew inspiration from Mandela, Obama mourned his death and said that the world was unlikely to see a leader like him again.
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"I am one of the countless millions who drew inspiration from Nelson Mandela's life. My very first political action -- the first thing I ever did that involved an issue or a policy or politics was a protest against apartheid," Obama told reporters soon after he was informed about Mandela's death.
UN Secretary-General Ban Ki-moon and the Security Council also mourned Mandela's demise, saying the South African leader was a "giant for justice" and a human inspiration who made great personal sacrifices in his life-long fight against racial oppression.
"Nelson Mandela was a singular figure on the global stage, a man of quiet dignity and towering achievement, a giant for justice and a down-to-earth human inspiration. I am profoundly saddened by his passing," Ban said in his condolence message.
Indians living abroad mourned the death of Mandela, who in his lifetime had said that Mahatma Gandhi was his inspiration. Prime Minister Manmohan Singh, in condoling Mandela's death hailed him as a "true Gandhian".
The group's chair, Kofi Annan, said the world had lost "a clear moral compass".