"Around 6am (4am 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) told a news conference in Harare.
Police spokesperson Charity Charamba confirmed the arrest just three days before key general elections, but did not elaborate immediately on the charge against Komichi.
"Yes, he has been arrested," she told Agence France-Presse.
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.
Chamisa accused the ZEC of 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 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."
Zimbabwe's elections on July 31 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 hadn't 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 didn't deliver a conclusive winner.
Mugabe and Tsvangirai were forced to form a power-sharing government a year later. – AFP