A Zimbabwean election commissioner has resigned, citing doubts about the integrity of results showing a big win for President Robert Mugabe’s ZANU-PF party but dismissed as a fraud-riddled farce by his main challenger.

Mkhululi Nyathi said he quit the nine-member Zimbabwe Electoral Commission (ZEC) over the way it managed the presidential and parliamentary vote on Wednesday. His resignation is likely to add to the dispute over the election both inside and outside Zimbabwe.

The vote, which looks certain to extend 89-year-old Mugabe’s 33-year rule in the southern African nation, passed off peacefully and received broad approval from African observers.

Africa’s oldest leader, Mugabe has governed the former British colony, then known as Rhodesia, since independence in 1980.

Mugabe’s main rival, Prime Minister Morgan Tsvangirai, has denounced the July 31 election as a “huge farce”, alleging massive rigging by ZANU-PF. Zimbabwe’s largest domestic observer group has also called the elections “seriously compromised”.

Contacted by telephone, Nyathi declined to discuss his concerns in detail.

Tsvangirai’s Movement for Democratic Change (MDC) said on Friday it could take to the streets to challenge ZANU-PF’s claim of a landslide victory, made less than 24 hours after the polls had closed on Wednesday.

Results for the parliamentary vote released so far by the ZEC show that ZANU-PF will have a two-thirds majority that would allow it to make constitutional changes. Zimbabwe state media is also reporting Mugabe is poised for a resounding win.

The Zimbabwe Electoral Commission has until tomorrow to release the official results of the presidential vote.

Tsvangirai has called on the African Union and the Southern African Development Community (SADC) to investigate the vote, calling it “null and void” and “not credible”.

But he faces an uphill struggle to convince the regional bodies, as their observers have already endorsed the election as free and peaceful, while acknowledging minor problems.