Officials from the group Al-Adl wa al-Ihsane (Justice and Charity) said one of its leaders, Mohammed Abdelli, and 181 other members were arrested.
The arrests took place in Oujda, 541km east of the capital Rabat, and in the small nearby town of Beni Modhar late on Thursday.
"Police stormed the two places where the members were meeting and arrested them before they emptied the offices of everything they found inside, including computers and books," an official from the group said.
Another official said: "A total of 148 brothers (Al-Adl members) were arrested in Rabat and three other cities on Wednesday and the previous two days."
"All those arrested were released later but the mass arrests were unprecedented since we launched an 'Open Doors' campaign weeks ago in several cities," said the official.
Neither official wanted to be identified.
One of them said the authorities sealed off the group's office in Oujda and posted policemen to prevent access to it.
"It is the first time the authorities took such decision to seal off an office. In the past, they deployed police discreetly to watch without intervening," he added.
"Police stormed the two places where the members were meeting and arrested them before they emptied the offices of everything they found inside, including computers and books"
Al-Adl wa al-Ihsane official
Al-Adl, which shies away from violence, is tolerated but not authorised as a political party.
Fathallah Arslane, Al-Adl spokesman, said the crackdown was spurred by what he called biased reports and comments in some local anti-Islamist newspapers about the group's activities.
Media have said the leadership of Al-Adl wa al-Ihsane, which is the biggest opposition group with an estimated 250,000 members, told followers to prepare for a Qawma (uprising) this year to establish a purist Muslim fundamentalist state.
"We are organising activities like the 'Open Doors' campaign so that more people will know us for what we are, rather than what some media say about us," Arslane said, dismissing the reports of a rebellion.
Another of Al-Adl's leading figure said the authorities had been eyeing the "Open Doors" campaign closely and moved to stop it when they saw it was arousing interest.
"The authorities were alarmed by the large number of people who showed an interest in listening to us and joining us. The authorities do not want that ahead of next year's parliamentary elections," he added.
Government officials were not immediately available to comment but analysts said the government pursues a containment strategy to trim Al-Adl's strength without confronting it.
Morocco has been wary over possible Islamist attacks since a series of suicide bombings hit foreign and Jewish targets in the coastal city of Casablanca on May 16, 2003, claiming 45 lives and wounding dozens.
King Mohammed IV pledged that the attacks would be the last to rock Morocco.
Morocco borders Algeria where violence between government forces and armed Islamic extremists has caused more than 150,000 deaths since 1992.