The arrests came one day after Zimbabwe police extended a ban on political rallies and protests in Harare which the opposition has likened to "a state of emergency".
Rallies and demonstrations were banned in four districts of the capital until June 23, police said.
Chamisa said that the meeting, which was not being attended by the MDC's leaders, did not fall under the police restrictions.
"This was not a street or open air gathering but a meeting in our own party offices to discuss civil issues and we are treated like an illegal or terrorist organisation," he said.
The government of Robert Mugabe, the president, has routinely used riot police squads to crush anti-government rallies in the southern African country, which is suffering from severe shortages of food and fuel.
In March, Morgan Tsvangirai, the leader of the MDC, and dozens of other opposition activists were badly injured after being arrested at a prayer meeting in the capital.
Mugabe accuses the MDC of being under the control of Zimbabwe's former colonial power Britain and trying to overthrow his government as punishment for seizing and redistributing white-owned commercial farms to landless blacks.