Zimbabwe police have released 84 opposition activists without charge but some were beaten, the opposition says.
The police in March said these arms belonged to MDC members
Movement for Democratic Change spokesman Nelson Chamisa said some activists had gone to hospital to determine the extent of their injuries.
A police spokesman had said Saturday's arrests were in connection with recent bombings around the capital, Harare.
The MDC denies any links to the blasts and condemned the arrests. Some 200 members were originally detained.
The police released 115 of them on Sunday and later freed the others.
Mr Chamisa said the police did not have a search warrant when they raided the Movement for Democratic Change (MDC) headquarters in central Harare.
He told the BBC that the meeting was held in party offices to discuss civil issues and insisted the MDC was doing nothing illegal.
The arrests came a day after police extended a ban on political rallies.
In March, scores of MDC activists, including party leader Morgan Tsvangirai, were severely assaulted in police custody, sparking international condemnation.
The Zimbabwe government accused the MDC of being behind the bombing of several police stations.
Following Saturday's arrests, police spokesman Supt Andrew Phiri told the state-owned Sunday Mail newspaper:
"Some suspects arrested in connection with recent petrol bombs have given us leads we are following up."
Mr Chamisa said those detained had been taken to the Law and Order division of the Central Police Headquarters.
South Africa is currently trying to negotiate a political solution to Zimbabwe's escalating crisis.
The BBC's Peter Greste in South Africa says diplomats say Zimbabwe is in danger of breaking into open conflict.
Mr Chamisa said the police raid showed the government's disdain for the diplomatic efforts.