It is difficult to overstate the significance of the life of Nelson Mandela.

There is a danger of sounding trite or platitudinous on these occasions, but the encomia heaped upon this one man are fully justified. He was in the truest sense a world leader and it is instructive for all of us to consider why.

As the figurehead of the global anti-apartheid movement, he was extraordinary; for holding to his principles even at the cost of nearly three decades in prison. But, on his release from jail, he delivered on almost impossible levels of expectation, offering a model of how to make progress despite grave historical hurts, moving forward through reconciliation.

For those who grew up during the Cold War in particular, he offered a profound antidote to cynicism about international relations, and remains a symbol that politics and power can still be wedded to principles and an ambition for social change.

His long journey from freedom fighter to the first black president of South Africa will be recalled as a chain of key moments

Mandela's words from the dock at the 1964 Rivonia trial resonate as powerfully now as they did almost 50 years ago: "During my lifetime, I have dedicated myself to this struggle of the African people. I have fought against white domination and I have fought against black domination. I have cherished the ideal of a democratic and free society in which all persons live together in harmony and with equal opportunities - an ideal I hope to live for but, if needs be, an ideal for which I am prepared to die."

Sentenced to life imprisonment, his treatment on Robben Island could not have been further from such ideals. Yet he remained true to them for 27 years.

His release in 1990 was celebrated across the globe and is now recognised in South Africa as a watershed moment for the country. His first speech was at a mass rally in KwaZulu-Natal, where he told supporters to "close down your death factories" and "throw your weapons into the sea".

The refusal to indulge in retribution extended to his personal life. Questions about whether he was bitter at the harshness of his treatment and the harassment of his then wife, Winnie, were always sidestepped as he focused on building a better future.

Something of the unique combination of charisma, human warmth and guile he employed to bring old enemies together in readiness for multi-racial democracy is illustrated in another landmark moment.

During the years of sanctions, the boycott against international sporting fixtures was one of the most powerful. For the 1995 rugby world cup to be played in South Africa just a year after Mandela became president was hugely important to the white community, unconvinced at the prospect of their country being run by a man they still regarded as a terrorist.

The team learned the new national anthem Nkosi Sikelel iAfrika and their victory was cheered by black South Africans who had previously made a point of supporting their opponents. And the black president in a green Springbok jersey was cheered by the largely white crowd. Such moments do not change generations of prejudice but they demonstrate that change can be positive.

Mandela astutely took on the role of catalyst. The African ideal of Ubuntu, that he was part of a larger community of all races, made him a freedom fighter against apartheid. This explains much: not only his ability to embrace the Afrikaner but probably also his retirement in 1999 after one only term as president, in marked contrast to so many African leaders who have clung on to power. It was also in itself a tribute to his achievement of a new South Africa in which a working, multiracial democracy could be taken for granted.

As his third wife, Graca Machel has said, Nelson Mandela is a symbol but he was not a saint. His commitment to political action meant little time for his family and destroyed his first and second marriages.

His initial support for his second wife Winnie despite charges of corruption was the result of guilt for the difficulties she had faced alone. The recent disputes about the reburial of some family members and commercial enterprises using his name may be the result of that strain but it is a distressing contrast to the dignity that was Mandela's hallmark.

For millions of ordinary South Africans he will always be Father of the Rainbow Nation, the embodiment of equal rights and a better future. And around the world, South Africa's first black president will remain a symbol of resistance to injustice: the award of the Nobel peace prize jointly with FW de Klerk the international seal on co-operation. In an interview to mark his 94th birthday a year ago Mandela said: "On my last day I want to know that those who remain behind will say: 'The man who lies here has done his duty for his country and his people'". Of that, there can be no doubt.

In South Africa, he will be mourned like no other. Although he made mistakes, such as his failure to take action against the Aids epidemic, as long as he lived, there was confidence the new dawn he had made possible would endure. Even though it is 14 years since he stood down as president and his long illness has prepared South Africans for a future without his guiding presence, his death will bring uncertainty. But his legacy spans the globe and the only fitting tribute is for people in every corner of the world to rise to the challenge he laid down.

His hope, explicitly stated, was that there would emerge "a cadre of leaders in my own country and region, on my continent and in the world, which will not allow that any should be denied their freedom, as we were, that any should be turned into refugees, as we were, that any should be condemned to go hungry, as we were, that any should be stripped of their human dignity, as we were".

"A better world is possible" has become a slogan for anti-globalisation campaigners. For generations of people internationally, Nelson Mandela has symbolised the truth of those words. But leaders of such protest movements should remember that Mandela's success in overcoming the old order was achieved in no small part because his moral certainty was backed by absolute humility.