Heavily armed police swooped down on opposition headquarters and independent election observers' offices on Friday, arresting hundreds and beating and shoving scores of people in the clearest signal to date that the government intends to hold on to power.

Police seized material on vote counting from both offices in raids that came a day after the United States declared opposition leader Morgan Tsvangirai had won the presidential elections. Zimbabweans are still awaiting official results. The opposition charges that President Robert Mugabe is using violence and coercion to hold on to power.

The opposition Movement for Democratic Change and the independent Zimbabwe Electoral Support Network both claim Tsvangirai won the vote, 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 arrest him and his deputy, Rindai Chipfunde-Vava, but that both were away from the office. He said they were both 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 after being attacked by ruling party loyalists.

"Their homes were burned," Thokozani Khupe, an MDC vice president said. "Some have been brutally assaulted."

Police said the officers raided Harvest House, headquarters of the opposition Movement for Democratic Change, in search of suspects responsible for the violence that has erupted in Zimbabwe in the wake of last month's disputed elections.

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."

Police could not immediately be reached for comment on the raid on the offices of the election observers.

Police took computers and equipment, and searched for key election-related documents, the MDC said.

The opposition and independent religious and human rights groups have accused Mugabe's regime of a violent crackdown on dissent since the poll almost a month ago.

Mugabe's officials have countered by accusing the opposition of violence.

Nearly four weeks after the elections, the presidential results — which the MDC claims show their candidate, Morgan Tsvangirai, as the victor — have not been published and a partial recount of the parliamentary elections continues.

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.

"We think in this situation we have a clear victor: Morgan Tsvangirai won, and perhaps outright," U.S. envoy Jendayi Frazer said during a visit to South Africa Thursday.

Frazer, assistant U.S. secretary of state for African affairs, is on a visit to southern Africa to raise international pressure on the government in Zimbabwe. She was in Angola for a meeting Friday with President Eduardo dos Santos, as was a delegation sent by Mugabe. Angola is a close ally of Zimbabwe.

She is also 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's delegation to Angola was led by Emmerson Mnangagwa, the feared former security minister once touted as Mugabe's possible successor. Mnangagwa also had led Zimbabwe's delegation to the recent regional summit on Zimbabwe in Zambia.

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.