Zimbabwean police have arrested Morgan Komichi, the election organiser of Prime Minister Morgan Tsvangirai's party after he reported marked ballot papers were found in a dustbin following early voting, the party and police said.
The arrest on Sunday came as thousands of President Robert Mugabe's supporters gathered at a national sports stadium in Harare for his final rally ahead of July 31 elections.
"Around 6:00am (04:00 GMT) our deputy national chairman, Honourable Morgan Komichi, who is a deputy minister of transport, was picked up at his home by the police," Nelson Chamisa from the Movement for Democratic Change (MDC) said.
Police spokeswoman Charity Charamba confirmed the arrest which came just three days before key general elections, but did not elaborate on the charge against Komichi.
Komichi this week handed an envelope with ballots marked for Tsvangirai to the Zimbabwe Electoral Commission (ZEC).
The MDC said several such ballots had been dumped in a dustbin at the Harare International Conference Centre after security forces, who will be on duty in Wednesday's polls, voted early on July 14 and 15.
Fears of vote-rigging
Chamisa accused the ZEC of launching a witch-hunt instead of dealing with the dumped ballot issue after the commission insisted Komichi be questioned over his source.
"There is no denial of the fact that indeed it's an authentic ballot paper and indeed that the ballot paper was found in the dustbin, but of course they want to know the whistleblower," 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."
Zimbabwe's elections pit veteran President Robert Mugabe, 89, against his rival Tsvangirai, 61, to end their uneasy four-year coalition government. Voters will also elect a new parliament.
Chamisa reiterated Tsvangirai's fears of a rigged vote.
"The development we have witnessed seriously demonstrates a dent on the credibility of this election. It demonstrates a perforation of the integrity of ZEC," he said.
The MDC, which won more votes in parliamentary elections in 2008 than Mugabe's Zanu-PF party, still had not received important election-related information, he added.
"The voters' roll has not been availed to us, we don't know who is printing the ballot papers. We don't know where those ballot papers are being printed," he said.
Violence erupted after the first round of the 2008 polls did not deliver a conclusive winner. Mugabe and Tsvangirai were forced to form a power-sharing government a year later.
The country is still emerging from a decade of economic decline and hyperinflation.