Zimbabwean police on Sunday arrested an aide to Morgan Tsvangirai, President Robert Mugabe's main rival in upcoming polls, after he reported an irregularity in early voting.

The Movement for Democratic Change said Morgan Komichi was arrested after informing the electoral commission that ballot papers marked for Tsvangirai had been found in a dustbin after security forces voted at the Harare International Conference Centre two weeks ahead of nationwide polling on July 31.

"Around 6:00 am (0400 GMT) our deputy national chairman, Honourable Morgan Komichi, who is a deputy minister of transport, was picked up at his home by the police," the MDC's Nelson Chamisa told reporters.

Police spokeswoman Charity Charamba confirmed the arrest just three days before general elections that will end an uneasy unity government between Mugabe, 89, and his archrival Prime Minister Morgan Tsvangirai.

"Yes, he has been arrested," she told AFP.

Komichi, Tsvangirai's delegate at the Zimbabwe Electoral Commission (ZEC), this week handed the polling panel an envelope with ballot papers in favour of Tsvangirai.

The MDC said several such ballots had been dumped in a dustbin at the conference centre where security forces, who will be on duty in Wednesday's polls, voted early on July 14 and 15.

Now police threaten to charge Komichi for contravening electoral laws if he refuses to reveal his source.

"As long as he refuses to disclose the identity of this person the police will hold him accountable, therefore he becomes the prime suspect," said Charamba.

"This is a very urgent and serious matter that will have bearing on the election process and therefore cannot be left for any other time except now."

The discovery of the papers added to fears of vote-tampering that Tsvangirai has raised in recent days.

But Mugabe branded Tsvangirai a "cry-baby," pledging the polls will be legitimate.

"It's going to be free and fair. We are not forcing anyone to vote this way or that way," Mugabe told reporters after addressing his final campaign rally in Harare.

Zimbabweans vote for a new president and parliament on Wednesday, four years after Mugabe, who has ruled the country since independence in 1980, and Tsvangirai were forced to share power.

"Vote, vote, vote in peace, peace peace, peace. We want peace," Mugabe told around 40,000 supporters at his rally at the National Sports Stadium.

But as campaigning wrapped up, the MDC's Chamisa accused the ZEC of launching a witch hunt instead of dealing with the dumped ballot issue.

"There is no denial of the fact that indeed it's an authentic ballot paper and indeed the ballot paper was found in the dustbin, but of course they want to know the whistle-blower," Chamisa said.

"We believe that ZEC and not Komichi have a lot of questions to answer," he added.

"If there is any investigation, the theatre of investigations is supposed to be ZEC."

"The development we have witnessed seriously demonstrates a dent on the credibility of this election," he said, questioning the electoral commission's neutrality.

Chamisa complained that the MDC, which won more votes than Mugabe's Zanu-PF in the 2008 parliamentary polls, has been denied important election-related information.

"The voters' roll has not been availed to us, we don't know who is printing the ballot papers," he said.

Violence erupted after the first round of the 2008 polls didn't deliver a conclusive winner. Mugabe said he was pleased that the election campaign has been devoid of violence so far.

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