Around 300 people were arrested on Friday after police raided the Movement for Democratic Change (MDC) headquarters in the capital Harare.
Tendai Biti, the MDC secretary-general, told Al Jazeera that the raid by about 250 riot police carrying batons was part of a government campaign against the opposition.
"It's a violent crackdown. They've arrested 360 people in our head office, about 300 of those people were already refugees fleeing violence and torture that is taking place in the countryside," he said.
"What is shocking at this present moment is that the UN Commission on Human Rights has not intervened in a situation that is clearly as unacceptable as it is heinous."
Police said the raid at the MDC headquarters had targeted people who had sought refuge there after they "committed crimes" outside Harare.
"Some of them are not office workers at all. We are busy screening them," Wayne Bvudzijena, a police spokesman, said.
"There are some cases we are investigating and we will release those who have not committed any crime."
Jacob Zuma, the head of South Africa's ruling African National Congress (ANC), condemned the raid, saying that it appeared "somebody is sabotaging the elections".
Zuma told the Associated Press news agency in an interview on Friday that the raid made the country resemble a police state.
Tensions have been running high since the presidential and parliamentary elections in Zimbabwe on March 29.
The partial recount has put the results of the parliamentary poll in doubt while the election commission is still to release any results from the presidential vote.
Morgan Tsvangirai, the MDC leader, has said that he won the presidential election outright and has accused Mugabe of delaying results to rig a victory.
The MDC and the main election observers - the Zimbabwe Electoral Support Network (ZESN) - both claim Tsvangirai won the poll, based on their own surveys of results posted at ballot stations.
The MDC said that police took computers and equipment during Friday's raid.
Noel Kututwa, chairman of the ZESN, said that officers from the Central Intelligence Organisation said they were looking for "subversive material likely to overthrow the government using unconstitutional means".
Kututwa also said that police wanted to arrest him and Rindai Chipfunde-Vava, his deputy.
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.
|Police carrying batons raided the MDC offices|
in the capital Harare [AFP]
The opposition and independent religious and human-rights groups have accused Mugabe's government of a violent crackdown since the polls.
Human Rights Watch said that a campaign against those perceived to have "voted wrongly" has escalated this week.
Carolyn Norris, the New York-based group's deputy director for Africa, said soldiers have joined in torturing and beating people in recent days.
Previously, ruling party officials, militiamen and war veterans carried out the violence at informal torture centres in the countryside, she said.
Mugabe's officials have denied organising or encouraging the violence.
Meanwhile, a Chinese ship carrying arms destined for Zimbabwe wasn allowed to dock at the Angolan port of Lobito, authorities said it would only be allowed to unload cargo for Angola.