|Former US President, George Clinton and nelson Mandela|
UN Secretary General Ban Ki-moon said he was "a giant for justice and a down-to-earth human inspiration".
"Many around the world were greatly influenced by his selfless struggle for human dignity, equality and freedom. He touched our lives in deeply personal ways."
US President Barack Obama said Mr Mandela achieved more than could be expected of any man.
"He no longer belongs to us - he belongs to the ages," he said, adding that Mr Mandela "took history in his hands and bent the arc of the moral universe towards justice".
Mr Obama, the first black president of the United States, said he was one of the millions who drew inspiration from Mr Mandela's life. He has ordered that the White House flag be flown at half-mast.
FW de Klerk, who as South Africa's last white president ordered Mr Mandela's release, called him a "unifier" and said he had "a remarkable lack of bitterness".
He told the BBC Mr Mandela's greatest legacy "is that we are basically at peace with each other notwithstanding our great diversity, that we will be taking hands once again now around his death and around our common sadness and mourning".
The Elders - a group of global leaders set up by Mr Mandela to pursue peace and human rights - said they "join millions of people around the world who were inspired by his courage and touched by his compassion".
The group's chair, Kofi Annan, said the world had lost "a clear moral compass".
"While I mourn the loss of one of Africa's most distinguished leaders, Madiba's legacy beckons us to follow his example to strive for human rights, reconciliation and justice for all."
UK Prime Minister David Cameron said "a great light has gone out in the world".
Earlier this year, Mr Mandela spent nearly three months in hospital with a recurring lung infection.
He was moved to his home in the Houghton suburb of Johannesburg in September, where he continued to receive intensive care.
Born in 1918, Nelson Mandela joined the African National Congress (ANC) in 1943, as a law student.
"Laura and I join the people of South Africa and the world in celebrating the life of Nelson Rolihlahla Mandela. President Mandela was one of the great forces for freedom and equality of our time. He bore his burdens with dignity and grace, and our world is better off because of his example. This good man will be missed, but his contributions will live on forever. Laura and I send our heartfelt sympathy to President Mandela's family and to the citizens of the nation he loved."
"[Nelson Mandela taught us so much about so many things. Perhaps the greatest lesson, especially for young people, is that, while bad things do happen to good people, we still have the freedom and responsibility to decide how to respond to injustice, cruelty and violence and how they will affect our spirits, hearts and minds."
"Barbara and I mourn the passing of one of the greatest believers in freedom we have had the privilege to know. As President, I watched in wonder as Nelson Mandela had the remarkable capacity to forgive his jailers following 26 years of wrongful imprisonment -- setting a powerful example of redemption and grace for us all. He was a man of tremendous moral courage, who changed the course of history in his country. Barbara and I had great respect for President Mandela, and send our condolences to his family and countrymen."
"Rosalynn and I are deeply saddened by the death of Nelson Mandela. The people of South Africa and human rights advocates around the world have lost a great leader. His passion for freedom and justice created new hope for generations of oppressed people worldwide, and because of him, South Africa is today one of the world's leading democracies. In recent years, I was gratified to be able to work with him through The Elders to encourage resolution of conflicts and advance social justice and human rights in many nations. We extend our heartfelt condolences to his family at this difficult time."
"Only because of such a great man like Nelson Mandela is it possible that particular people in Africa and elsewhere are able to enjoy freedom and human dignity. We have to learn the wisdom and determinations and commitment of Mr Mandela to make this world better for all."