Group members on Wednesday said security forces have arrested more than 500 members of Al Adl wa al Ihsane (Justice and Charity) since late May after it launched an "open doors" campaign to recruit outside traditional areas such as mosques and universities.
Some were beaten and nearly all were quickly released.
Fathallah Arslane, Al Adl's spokesman, told Reuters that 45 group activists were arrested in the town of Bouarfa in northeast Morocco on Tuesday night before being set free in the early hours of the morning.
He said 43 Al Adl members were rounded up earlier in Oujda and Nador, also in the northeast, among them the group's second-in-command, Mohamed Abadi.
All but one were released, although Abadi and two others must face prosecutors at the end of the month.
"The authorities want to limit the group's activities, not destroy it," said Mohamed Darif, an Islamism specialist at Hassan II university in Mohammedia near Casablanca.
Arslane said: "The authorities want to muzzle us."
Al Adl's founder, Abdessalam Yassine, was under house arrest for almost 10 years until 2000 for challenging the monarchy's powers, including the king's status as Commander of the Faithful - the spiritual leader of the country's Muslim community.
The group, Morocco's biggest opposition force with about 250,000 members, would like to see an Islamic state organised according to sharia law, although it says it rejects violence.
In the short term it says it is fighting for reforms including trimming the power of the royal palace and giving more authority to the prime minister.
But Chakib Benmoussa, the interior minister, said this week the group's unauthorised activity had put it "outside the logic of the law" and that "it is the duty of the state to enforce scrupulously the law".
Observers say the scale of popular support for Al Adl would make it a force to be reckoned with were it ever allowed to enter mainstream politics.
Its leaders prefer to remain aloof, insisting they will stay on the sidelines unless the government reforms the constitution.