Riot police in Zimbabwe have carried out raids on headquarters of independent poll monitors and the opposition MDC in the capital, Harare.
Witnesses say vote-counting material was taken from the MDC office and activists hiding there were arrested.
The Zimbabwe Election Support Network chairman told the BBC that documents and computers had been seized.
The observer group says MDC leader Morgan Tsvangirai gained the most votes in last month's presidential election.
Officials results have not yet been released.
The Movement for Democratic Change (MDC) says Mr Tsvangirai won the election outright, while the Zimbabwe Election Support Network (ZESN) says he fell just short of the 50% threshold to avoid a run-off.
The ruling Zanu-PF party also says there is likely to be a run-off, as no candidate gained more than 50% of the vote.
'They took everyone'
Witnesses at the MDC raid said at least 100 opposition supporters who had been taking refuge from the authorities in its Harvest House headquarters had been arrested.
"They are trying to destroy evidence of their brutality"
Computers and documents were also seized, they said.
MDC spokesman Nelson Chamisa said the activists had fled political violence.
"They took everyone in the building, including those who had come just to seek medical care. They are trying to destroy evidence of their brutality," Mr Chamisa said.
But police said the aim of the raid was to find those responsible for arson attacks east of Harare.
Spokesman Wayne Bvudzijena said police were screening those detained and anyone who had not committed any crimes would be freed.
ZESN chairman Noel Kututwa told the BBC's Focus on Africa programme that the police had a search warrant to look for "subversive information likely to overthrow a constitutionally elected government".
He said that no-one had been arrested but the body's programme manager had been asked to go the police station to explain the role of the network.
He also said he had gone into hiding.
ZESN was the largest observer group at the 29 March election and is considered the only reliable source of information about the polls, correspondents say.
'Myopic stooges'
The MDC says its activists have been attacked around the country - with at least 10 killed - since the elections.
Many have fled to Harare and other towns, seeking medical treatment.
But the police and Zanu-PF say that no-one has died in political violence.
MDC leader Morgan Tsvangirai has fled Zimbabwe, fearing for his safety and is touring African countries, trying to persuade them to press President Robert Mugabe to step down.
The electoral commission says it cannot release the presidential results until it completes a recount in 23 of the 210 constituencies.
Three recounts of the parliamentary results have been completed - all confirmed the original results.
President Robert Mugabe's Zanu-PF party lost control of parliament for the first time since independence in 1980.
But if many of the results are overturned in the recount, this could change.
Meanwhile, the Herald newspaper has condemned Zimbabwe's neighbours as "myopic stooges" for refusing to let a cargo of Chinese weapons cross their territory to landlocked Zimbabwe.
"Zimbabwe is... under attack from the former coloniser and its allies. As such, Zimbabwe probably needs to arm itself more than any other country in Africa today for the simple reason that it has been targeted for destabilisation by the traditional Western rabble rousers," the Herald said.
China's foreign ministry says the ship will now return as it cannot deliver its cargo to Zimbabwe.
But the state-owned shipping company has not confirmed this.
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