Nelson Mandela has died aged 95 at his home.
The former South African president had been ill for some time and was admitted to hospital earlier this year with a recurring lung infection.
President Jacob Zuma made the announcement, saying: "He is now at peace".
In a televised address, Zuma said: "Our nation has lost its greatest son. Our people have lost a father.
"What made Nelson Mandela great was precisely what made him human. We saw in him what we seek in ourselves."
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David Cameron tweeted his condolences, adding: "A great light has gone out in the world. Nelson Mandela was a hero of our time. I've asked for the flag at No10 to be flown at half mast."
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Zuma said: "Fellow South Africans, our beloved Nelson Rolihlahla Mandela, the founding president of our democratic nation has departed.
"He is now resting. He is now at peace.
"Our nation has lost its greatest son. Our people have lost a father."
Zuma said all of South Africa's thoughts were with Mr Mandela's family, friends and those who fought alongside him during his struggle for equality.
"Our thoughts are with the South African people who today mourn the loss of the one person who more than any other came to embody their sense of a common nation," he said from Pretoria.
"Our thoughts are with the millions of people across the world who embraced Madiba as their own and who saw his cause as their cause.
"This is the moment of our deepest sorrow. Our nation has lost his greatest son. Yet what made Nelson Mandela great was precisely what made him human - we saw in him what we seek in ourselves and in him we saw so much of ourselves."
Zuma said the man known as Madiba brought South Africa together and would be afforded a state funeral. South African flags would be lowered to half mast from tomorrow until the funeral, he said.
"As we gather to pay our last respects, let us conduct ourselves with the dignity and respect that Madiba personified," he said.
"Let us be mindful of his wishes and the wishes of his family as we gather wherever we are in the country and wherever we are in the world.
"Let us recall the values for which Madiba fought.
"Let us reaffirm his vision of a society in which none is exploited, oppressed or dispossessed by another."
Zuma finished his address by saying simply: "We will always love you Madiba. May your soul rest in peace. God bless Africa."