The streets of Harare were largely empty yesterday as a nervous nation digested its official election result and awaited the coronation once again of its ageing, cancer-ridden ruler. No one should be under the slightest delusion Zimbabwe has held a free, let alone remotely fair, ballot. This was daylight robbery, carried out under the gaze of the globe and rubber-stamped by Robert Mugabe's useful idiots.

As the result was declared yesterday, it was hard to think of a national election in recent years stolen so blatantly. Not for nothing were election observers from the United Nations, the European Union and the respected Carter Centre kept out, while those from undemocratic nations such as Belarus, China, Cuba, Russia, Iran and Sudan were welcomed. The key question now is how Zimbabwe and the international community reacts to such ruthless ballot-rigging.

So effective was the vote-fixing that even some of Mugabe's aides were shocked by the scale of their triumph. Nothing was left to chance. The plan was laid down after the 2008 election, when the old man was rejected amid grinding poverty and grotesque hyper-inflation, clinging to the presidency only after his generals unleashed extreme violence on the opposition Movement for Democratic Change (MDC). Eventually, foreign diplomats forced an uneasy coalition on a wounded country, which stabilised Zimbabwe largely through the introduction of the dollar as the official currency.

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The aim this time was to avoid the violence that attracted worldwide attention in 2008 – although militia were recruited and torture centres put on standby just in case. As the election loomed, civic groups had legal cases hung around their necks. Indoctrination and intimidation were stepped up, with people warned they would lose their jobs, homes and land rights if they did not vote for Mugabe. Overtures were made to key officials in southern African countries sending election monitors. Most importantly, the electoral roll was manipulated with the aid of a shadowy Israeli firm paid $13m (£8.5m).

All this was laid out in secret security force documents I obtained in Harare a fortnight ago. They also, incidentally, showed dwindling support for Mugabe's Zanu-PF party, despite propaganda pumped out by state-controlled media. The Chinese-backed masterplan was underlined by independent auditors, showing that in 63 constituencies there were more people registered to vote than residents, while there were 109,000 centenarians on the list and younger voters being excluded.

The MDC, having walked like zombies into Mugabe's trap, called the vote a farce. They are backed by the biggest group of citizen monitors, which reported systematic efforts to disenfranchise up to one million urban voters. One Western diplomat found a woman with nine dead relatives on the register; he saw another told to vote 200 miles away. Security sources told me that 750,000 people were turned away after names were removed or placed in other constituencies.

Even the African Union, a body currently chaired by the ruler of a repressive one-party state, had "great and grave concerns" – although the head of its mission, Nigeria's former president Olusegun Obasanjo, curiously rushed to back the result. The MDC seemed shell-shocked; one person who met leader Morgan Tsvangirai afterwards was stunned by his defeatist attitudes. As ever, dark rumours circulate with each twist in Zimbabwe's surreal saga; often, they turn out to be true.

The final verdict of monitors from the Southern African Development Community is vital; they described the elections as "free and peaceful" but carefully said it was too early to call them fair. Yesterday, Tsvangirai called on them to investigate as he announced probably futile plans to challenge the result in court. His stance will help Zimbabwe's neighbours such as South Africa and Botswana who have concerns – but the Zanu-PF masterplan earmarked significant resources to neutralise just such a threat. Meanwhile, Mugabe's goons may hunt down people who might testify about ballot-rigging; already evidence is emerging that retribution against witnesses has begun.

If Mugabe gets away with his theft, and a highly-educated population suffering hardship and huge unemployment stays off the streets, what happens next? Jostling is expected at the top of Zanu-PF for control of the country and its lucrative mines. The hardliners under defence chief Emmerson Mnangagwa – whose sticky fingers were first seen in the Congolese wars more than a decade ago – strengthened their position in the election; he looks in pole position to succeed the ailing 89-year-old president. One moderate Zanu-PF activist told me last night the mood felt funereal.

The West is left in a difficult position – especially Britain, the former colonial master and butt of much of Mugabe's stale rhetoric. Most of the sanctions imposed on Zimbabwe were lifted earlier this year as an incentive for stuttering political reforms; they remain, however, a useful political prop for Zanu-PF to rail against, as shown in the short campaign.

Since Britain supports other equally revolting regimes and engagement is preferable to isolation, the smart thing now would be to lift remaining sanctions, turn off the aid taps and challenge Mugabe to deliver his promises on public services. It is risky: his "indiginisation" policies could shatter the economy again. But he will need to reassure investors and only then might mining proceeds be diverted into state coffers instead of the pockets of generals.

In truth, Mugabe and his government of gangsters seem again to have outplayed their rivals, both at home and abroad, retaining their iron grip on power with brazen dirty tricks funded by diamond profits. It is hard to see any other path to support the biggest losers in last week's electoral larceny –the people of Zimbabwe, who have already suffered more than enough.