The start of the World Cup this Friday will be a moment to savour for South Africa. Just over 20 years since Nelson Mandela walked to freedom, the people of the so-called "rainbow nation" will bask in the admiration of the world not only for their success in hosting such a huge event but, in a broader sense, for having maintained their country as an oasis of democratic values and an economic power in southern Africa.

No one should want to rain on South Africa's parade. At the same time, it is important not to forget the existence of countless people there for whom the World Cup will mean almost nothing. That includes not only most poor black South Africans, one quarter of whom are unemployed, but the five million black migrants who have fled there in recent years and who often eke out a desperate existence, marked not only by poverty but by extreme violence on the part of South Africans who resent their presence and economic competition.

As we report today, the plight of about three million Zimbabweans in South Africa, economic and political refugees from Robert Mugabe's abysmally mismanaged fiefdom, is particularly worrying. Some organisations are expressing fears that the xenophobic attacks to which they are routinely exposed are merely being halted by police for the duration of the World Cup, after which they will resume as usual. A wave of such attacks on migrants in South Africa's shanty towns, which left more than 60 dead and displaced tens of thousands in 2008, briefly captured international attention. But the spotlight soon moved on, since when the violence has continued, albeit at a lower intensity.

The South African government's relationship with this exiled Zimbabwean diaspora is ambiguous, reflecting the strange pas de deux between the old tyrant in Harare and two successive South African presidents, Thabo Mbeki and now Jacob Zuma, who out of misplaced racial solidarity have never openly criticised the Mugabe regime or drawn attention to the fact that one of the many bad results of Mugabe's rule has been the mass flight of Zimbabweans across the border.

An eerie official silence continues to envelop the whole issue of this influx, which only fuels the violence against the migrants, for poor black South Africans have never been told why it is occurring. Many people who feel only goodwill for South Africa are rightly puzzled. As the World Cup confirms South Africa's place on the world stage, it is time for its leaders to acknowledge that with power comes responsibility to show leadership.