About 300 people were beaten and forcibly arrested in the raid, party officials for the Movement for Democratic Change told the Associated Press.
"These armed police have taken hundreds of people that were now staying at the party headquarters running away from the different parts of Zimbabwe, where the regime has been unleashing brutal violence," an MDC statement said, reported Reuters.
Spokesman Nelson Chamisa told Reuters: "They are trying to destroy evidence of their brutality."
The offices of independent election observers were also reportedly raided and materials on vote counting were seized from both offices. Zimbabwe has still not released results from the March 29 election, which the MDC and independent observers claim the opposition won, based on compiled results posted at individual ballot stations.
The results of the parliamentary elections are also in question. The opposition defeated Mugabe's party and won the majority of seats in last month's election. But a recount of some of the votes threatened to overturn those results.
Of the 23 constituencies holding a recount, only two did not go to the MDC originally. But the nine tallies that have been recounted this week have not produced any new victors, giving Mugabe's ZANU-PF party no new wins.
The rest of the results should be released by the weekend, according to government officials.
Opposition leader Morgan Tsvangirai accuses longtime leader Mugabe, 84, of delaying results to stay in power in Zimbabwe, whose economy lies in ruins with soaring inflation and chronic food and fuel shortages.
The government-run newspaper reported Wednesday that no one party would have enough votes to declare a victory in the presidential election, and suggested a national unity government could be the only solution.
"The Zimbabwe government and independent international observers are agreed that the just-ended harmonized elections did not produce an outright winner in the presidential race. It is unlikely the ongoing recount will substantively alter that position," the article said, reported AFP.
"Accordingly, it stands to reason that the transitional government of national unity ... should be led by the incumbent president."
The newspaper also called African leaders who have echoed Western criticisms of Mugabe "myopic stooges."
The top U.S. diplomat for Africa, Assistant Secretary of State Jendayi Frazer, said Thursday the United States is backing a British call for an international arms embargo against the country.
Concern over arms flared up last week when a Chinese weapons shipment headed to Zimbabwe was turned away by ports in southern Africa. That shipment is now being returned, reported the Los Angeles Times.