Zimbabwe's disputed election has plunged the country back into a deep political crisis and could open the way for decades more of autocratic rule.

"At least it was peaceful" was the meek assessment of one Harare-based diplomat after Wednesday's presidential poll.

Even before the final votes had been counted, Robert Mugabe's allies claimed a comprehensive victory that would extend his 33-year rule.

The 89-year-old has seemingly performed a political miracle, winning by a landslide despite years of crippling unemployment that forced millions to emigrate.

The African Union quickly declared the vote free and fair, dismissing widespread allegations of rigging.

But independent observers said as many as one million people were prevented from voting in opposition urban strongholds, while Mugabe's support was inflated by repeat and "ghost" voters.

In the run-up to the vote, state media and the security services appeared to be an extension of Mugabe's campaign machine.

Mugabe's long-time rival Morgan Tsvangirai, leader of the Movement for Democratic Change (MDC), cried foul, describing the vote as "a sham."

But the scale of the supposed rout left many MDC members in a state of stupor.

"I'm totally shocked at how badly they've rigged it," said MDC stalwart Roy Bennett.

Many in the MDC had hoped a vast turnout would make rigging impossible, and Mugabe, who has been in an uneasy power-sharing agreement with Tsvangirai, would be made to step aside by sheer force of opposition.

Instead the MDC is left fighting for its own political life.

"If indeed they have lost then they have to rebrand and have leadership renewal. They might start asking if they should continue to have Tsvangirai as leader," said Dumisani Nkomo, a political analyst based in Bulawayo.

It may yet get worse for the party.

"It's going to be very difficult for the MDC to make a comeback with fewer members in parliament," said Shakespeare Hamauswa, a political scientist at the University of Zimbabwe.

Official results may give Mugabe's ZANU-PF a two-thirds majority in parliament.

That would be enough to rewrite a new constitution overwhelmingly approved by Zimbabweans in March, which introduced term limits and curbs on presidential powers.

Control of the presidency and parliament would then give Mugabe's party breathing room to choose a successor.

Throughout his rule, Mugabe has steadfastly refused to name who should succeed him.

But behind the scenes there is fierce jockeying within ZANU-PF's rival camps, one led by Vice President Joice Mujuru and the other by hardline Defence Minister Emmerson Mnangagwa.

If Mugabe is declared winner, one of his first tasks will be to name a vice president, offering a hint of who might win that tussle.

Tsvangirai may have little option but to stand by and watch.

Legal efforts to annul the vote are likely to run into a wall since the courts are packed with Mugabe's acolytes.

Firebrands who have hit out at the vote talk of filling Harare's streets with protesters and staging an "African Spring," but even calls for a campaign of passive resistance seem a tough ask.

"There needs to be resistance against this theft and the people of Zimbabwe need to speak out strongly," said the MDC's Bennett.

"I'm talking about people completely shutting the country down - don't pay any bills, don't attend work, just bring the country to a standstill," said the veteran opposition leader.

Cynics point to the result of similar demonstrations in 2008, which were quickly and brutally crushed, and the fact that Zimbabweans live from day-to-day -- skipping work is not an option.

The MDC will seek some recourse from neighbours.

SADC, the southern Africa regional bloc that has spent much of the last decade trying to stabilise Zimbabwe, and which observed the vote, is widely expected to give the election a pass when it reports on Friday.

Botswana's assertion that the vote "fell short of best practice" is unlikely to be replicated by the bloc as a whole.

SADC's dominant power South Africa has long shied away from publically criticising Mugabe.

Fearing another wave of Zimbabweans across its border, Pretoria may seek to extend the power-sharing government between Tsvangirai and Mugabe which while uneasy, at least put the political crisis on ice for four years.

After five long years of talks, agreements, power-sharing, constitution-writing and elections, Zimbabwe may be back where it started, with Mugabe firmly in the driver's seat.

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