Amnesty International said it had evidence to suggest at least 120 people were briefly detained last week after security forces beat protesters during a sit-in at the main square. Riot police prevented other demonstrations from reaching the city center in Casablanca.
The violence followed an announcement from Moroccan King Mohamed VI that the government was willing to protect political freedoms and undertake reforms to address protester demands.
The international rights group said "unnecessary acts of violence" in Morocco were a slap in the face to the promises of reform.
More than 1,000 people are expected to take part in weekend demonstrations, though reformers told Amnesty International there were growing fears security forces may resort to violence to quiet the protests.
"Moroccans must be allowed to peacefully protest without fear, rather than face attacks by security forces just for demanding their rights," Philip Luther, Amnesty International's deputy director for the Middle East and North Africa, said in a statement.