The world's most revered elder statesman arrives in London tomorrow to take part in the global celebrations of his 90th birthday (see page 36).

Nelson Mandela, the towering South African freedom fighter, who spent 27 years in prison, cast off the shackles of apartheid in 1990 and went on to lead his country for five momentous years. He remains in old age the most influential figure on the African continent. His name has become synonymous with the qualities he has, in his long life, exemplified: magnanimity, tolerance, compassion, wisdom and moral courage. In London, the Nobel prize-winner's life will be honoured with a star-studded concert in Hyde Park, a symbolic re-creation of the Free Mandela concert in Wembley Stadium 20 years ago that did much to raise global pressure for his release. Precisely 46,664 tickets are being sold, representing Mr Mandela's long Robben Island ordeal. He was prisoner 466, imprisoned in 1964.

Mr Mandela announced some years ago that he had “retired from retirement”. But although he travels and speaks less, the demands on his time, his name and his authority are ceaseless. He has frequently credited Gandhi as the main source of inspiration in his life. Now he too has become a figure of transcendental inspiration, a man of giant moral authority, whose example is held up across continents and conflicts. Two statues honour him in London. He has received more than 100 awards over four decades. There have been films of his life. A Broadway musical is soon to be staged. Monarchs, presidents and statesmen line up to meet him.

Such acclaim is neither inevitable nor necessarily welcome. Mr Mandela remains, amazingly, on a US list of terrorists because of the past record of violence of the African National Congress. A Bill is going through Congress to remove his name, but his past involvement in violent resistance is, as he admits, a fact of history. And mawkish attempts to sentimentalise the old man's life tend to hide both the hard moral choices he had to make, his lifelong opposition to repression whatever the skin colour of the oppressor and the danger that his name and legacy will be misappropriated by those who share little of his outlook and none of his principles.

Mr Mandela is coming to London not only to celebrate a long life but also to raise awareness of Aids in Africa and funds to fight the epidemic. Aids is an issue on which he took an early and outspoken lead in speaking out. On another issue, however, he has been woefully silent. Since his retirement, and indeed even before that, he has said little about the tragedy unfolding in Zimbabwe. There may be something of the old freedom fighter's anti-colonial instincts at work here; but Mr Mandela is in a unique position to fight once again for the cause - freedom and justice in Africa - that earned him the admiration of the world.

Mr Mugable and his henchmen could not ignore a rebuke from Mr Mandela, a man who endured more than they did in the name of black empowerment. He could also inspire the people of Zimbabwe with real courage and hope. And in the process, he would help to restore the authority of South Africa, a nation that for so long clamoured for support in its time of need and which has in recent months offered limp diplomacy as its neighbour endured starvation, intimidation and murder.

His aides insist that he does not now intervene in political issues, and that he needs in any case to conserve his strength. The excuse is feeble. They know that only a few sentences could change perceptions, galvanise Africa's attitude to the tyrant in its midst and spare thousands of lives from ruination. It is time Mr Mandela spoke out, here in London this week.