As Robert Mugabe basked in his own infamy in Rome on Wednesday, his enforcers in Zimbabwe soaked five opposition members with petrol and set them alight. Two died.

As the world food summit heard Mr Mugabe blame Britain for his people’s hunger, his Government suspended charity operations that have been keeping four million of them alive. As the UN counted the cost of Mr Mugabe’s diplomatic immunity yesterday, thugs attacked a convoy of British and US diplomats. Their crime: investigating the plague of state-sanctioned violence that grips Zimbabwe.

Yesterday’s incident at a roadblock north of Harare was inexcusable. But it was, as David Miliband noted hours later, merely a glimpse of the intimidation that Zimbabweans suffer daily for daring to demand change.

Their chance comes in 21 days, in a second-round presidential vote that by rights should not be happening. The first round, three months ago, was marred by murder, beatings and fraud but was still won by Morgan Tsvangirai, the man who offers the best hope of an end to Mr Mugabe’s 28-year tyranny. Mr Tsvangirai could have refused to fight a second round, but he has not. He had every right to insist on a truly international corps of election observers, but the regime has let in only monitors from Zimbabwe’s neighbours. He has called this “sufficient”.

The Movement for Democratic Change, which Mr Tsvangirai leads, has seen its rallies banned, its meeting venues occupied by army tents and its supporters killed and tortured by loyalist militias. He has survived three assassination attempts, and this week was detained for eight hours while campaigning near Bulawayo. He shrugged off the harassment and yesterday continued his efforts to unseat Mr Mugabe at the ballot box, accusing his rival, with little exaggeration, of turning Zimbabwe into a warzone.

Mr Tsvangirai has shown extraordinary courage in a struggle that he is by no means guaranteed to survive. Meanwhile, Mr Mugabe’s insulation from the appalling reality he has created, and Africa’s dismaying acquiescence, has persuaded him the world is powerless to stop his crimes.

It is not. If Zimbabwe’s neighbours and their Western partners can agree on the demands of basic justice, jettison the inhibitions of the past and co-ordinate their efforts in the coming weeks and months, they can help to end this nightmare. These are big ifs, mainly because of South Africa’s woeful failure to lead. President Mbeki’s refusal to condemn Mr Mugabe outright or enforce meaningful sanctions on his Government has deprived the international community of its best levers against Harare. But there are others.

Ban Ki Moon, the UN Secretary-General, has so far confined his public efforts in this crisis to vague calls for an end to violence and for international election observers. In Rome, he “suggested” that Mr Mugage receive a special UN envoy. He must stop suggesting and demand that Harare accept an envoy with the task of listing in stark terms the consequences of attempting to steal the June 27 vote. These could and should include the pursuit of foreign assets held by Mr Mugabe and his inner circle; the collection of evidence against them for potential use in criminal charges under the UN Convention on Torture; and, in extremis, a resolution allowing the freezing of foreign remittances on which Zimbabwe’s devastated economy depends.

The US, which currently chairs the UN Security Council, should make clear to Zimbabwe’s neighbours that a whitewash of election observers’ reports, which many fear, would be unacceptable. Whatever happens to Mr Mugabe this month, this is his endgame. The civilised world must use every legal means to win it.