Police seized material on vote counting from both offices. Some 200 people were arrested in the raid on opposition headquarters, according to party officials.
The raids came a day after the United States declared opposition leader Morgan Tsvangirai had won the presidential elections - from which Zimbabweans are still awaiting official results amid charges that President Robert Mugabe is using violence and stealth to hold on to power.
However, the U.S. State Department on Friday backed off those comments Friday.
"I'll just say that there is clearly a vote for change there and there are a lot of indications that he may well have won," said State Department spokesman Sean McCormack.
The opposition Movement for Democratic Change and the independent Zimbabwe Electoral Support Network both claim Tsvangirai won the poll, based on their own surveys of results posted at ballot stations.
A ZESN board member, who witnessed the raid and spoke on condition of anonymity for fear of arrest, said police ransacked files, looking for documentation on the results.
Noel Kututwa, chairman of the organization, said police wanted to arrested him and his deputy, Rindai Chipfunde-Vava, but that both were away from the office. He said they were in hiding and accused police of trying to intimidate the group so it would be too weak to monitor a possible run-off.
"They said they were looking for subversive material likely to overthrow government using unconstitutional means," Kututwa told The Associated Press.
The MDC said in a statement that some 250 heavily armed officers raided the building, taking away some 300 people, including staff members.
The opposition said most of those arrested - including pregnant women and mothers with small children - had been seeking refuge in the opposition headquarters after being attacked by ruling party loyalists.
"Their homes were burned," said Thokozani Khupe, an MDC vice president. "Some have been brutally assaulted."
Police said the officers raided Harvest House, MDC headquarters, in search of suspects responsible for postelection violence.
Assistant Police Commissioner Wayne Bvudzijena said he could not say how many people were rounded up. He said the suspects were responsible for "crimes that were committed in the countryside."
The MDC said officers took computers and equipment, and searched for key election-related documents.
Police comment on the raid on election observers' offices was not immediately available.
The opposition and independent religious and human rights groups have accused Mugabe's regime of a violent crackdown on dissent since the poll nearly a month ago.
Mugabe's officials have countered by accusing the opposition of violence.
The opposition has accused Mugabe of withholding the presidential election results while he plots how to keep power, and says he is orchestrating a campaign of retribution that the MDC says has killed at least 10 of its supporters.
U.S. envoy Jendayi Frazer said during a visit to South Africa that it was clear Tsvangirai won.
"We think in this situation we have a clear victor: Morgan Tsvangirai won, and perhaps outright," she said Thursday.
On Friday, McCormack, the State Department's spokesman, would not go as far as Frazer. He said questions about the U.S. interpretation of the election results should not distract international attention from the real problems in Zimbabwe.
"These comments not withstanding, the focus should really be on the really intolerable actions of the Mugabe regime," he said.
It is not the first time that the State Department has backtracked after comments by Frazer.
On Jan. 30, she described post-election violence in Kenya as "ethnic cleansing" while speaking to reporters. McCormack later distanced the United States from the emotionally charged phrase, refusing to repeat it and saying it was too soon to characterize the situation in such terms.
Frazer, assistant U.S. secretary of state for African affairs, is touring southern Africa to raise international pressure on Zimbabwe.
She was in Angola to meet Friday with President Eduardo dos Santos and is scheduled to travel to Zambia for talks with President Levy Mwanawasa, the current head of the Southern African Development Community of 15 nations, which is thought to have some sway over the intransigent Zimbabwean leader.
Mugabe also sent a delegation to Angola, a close ally of Zimbabwe, led by Emmerson Mnangagwa, the feared former security minister once touted as Mugabe's possible successor.
Details on the message Mugabe was sending via the Mnangagwa delegation were not immediately available. The delegation was scheduled to meet with Dos Santos shortly before Frazer's talks with the Angolan leader.