Police said the raid targeted people who had sought refuge at the opposition headquarters 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, said.
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.