Tendai Biti, the MDC secretary-general, told Al Jazeera the raid 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 targeted people who had sought refuge there after having "committed crimes" outside Harare.
"Some of them are not office workers at all. We are busy screening them. There are some cases we are investigating and we will release those who have not committed any crime," said Wayne Bvudzijena, a police spokesman.
Zimbabwe has been gripped by a post-electoral stalemate since polls last month.
No results have been announced for the March 29 presidential vote, while the outcome of a parliamentary poll which the MDC won, is also in doubt because of partial recounts.
Morgan Tsvangirai, the MDC leader, has said he won the presidential election outright and has accused Robert Mugabe, Zimbabwe's president, of delaying results to rig victory and keep his 28-year hold on power.
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.
Noel Kututwa, ZESN chairman, said police wanted to arrested him and Rindai Chipfunde-Vava, his deputy, but that both were away from the office at the time of the raid.
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," the Associated Press reported Kututwa as saying.
The opposition and independent religious and human rights groups have accused Mugabe's government of a violent crackdown since the polls.
Mugabe's officials have countered, also accusing the opposition of violence. The raids come a day after the US declared that Tsvangirai had won the presidential elections.
Speaking at an international trade fair in the opposition heartland of Bulawayo on Friday, Mugabe rejected foreign criticism of his country and defended his government's policy of seizing white-owned farms.
In Pretoria, South Africa's capital, dozens of Zimbabweans were arrested outside the Chinese embassy on Friday, during a protest over a ship that attempted to offload weapons destined for Zimbabwe.
|Dozens were arrested at the protest outside|
the Chinese embassy in South Africa [EPA]
China had ordered the ship to return home after port workers in South Africa refused to unload its cargo last week. However, it travelled on to Mozambique and Angola and was allowed to dock by Angola on Friday although it will not be allowed to unload its cargo.
Inspector Malusi Msimang, a South African police spokesman, said of the Pretoria demonstration: "A total of 129 protesters were arrested staging an illegal protest."
"The police asked them to disperse and they refused so, they were arrested.
"After their arrest, immigration officers were invited to determine their status. Only 18 of them are legally living in South Africa. The rest are illegal immigrants."
Msimang could not confirm if those arrested would be deported or charged with illegal assembly.