Marches are planned in several cities to push calls for urgent reforms and limits on the king's power, echoing protests for democracy under way across the Arab world.
"Thousands are expected to take to the streets across the country to demand political and human rights reform but there are fears the authorities may resort to heavy-handed tactics to quell protests," Amnesty said.
Security forces broke up a protest in the country's biggest city, Casablanca, on Sunday last week.
Scores of people were injured and at least 120 briefly detained when they used "unjustified force", the world human rights watchdog said in a statement that cited people wounded in beatings by security officers.
"The unnecessary acts of violence witnessed last weekend are a disturbing regression and make a mockery of the Moroccan king’s promise a few days earlier to undertake fundamental reform and uphold human rights," said Amnesty regional deputy director Philip Luther.
King Mohammed VI last month announced sweeping democratic reforms including an elected prime minister and broader personal freedoms in his first speech to the nation since the first demonstrations on February 20.
"The Moroccan authorities must uphold the right of protestors to express their views and demands in a peaceful manner and to instruct security forces not to resort to unjustified or unnecessary force to disperse demonstrations in line with Morocco’s international human rights obligations and the king’s promises," Luther said.