The February 20 movement, which is pushing for democratic reform in Morocco, on Monday "strongly condemned" the crackdown on a demonstration in Casablanca in which dozens of people were hurt.
The largely youthful movement also called for further demonstrations to go ahead as planned on March 20 in several cities to press for sweeping political changes.
"The (king's) speech of March 9 was a first gain for the Moroccan people because he announced new constitutional reforms. But it was followed by a process of repression against our movement which we strongly condemn," the movement said in a statement.
Dozens of people were injured, some of them seriously, in Casablanca on Sunday, when police tried to storm the headquarters of a left-wing political party and cracked down violently on a demonstration for change called for by the February 20 movement.
In a landmark speech last Wednesday, Mohammed VI announced major democratic reforms and "the enlargement of individual freedoms". It was his first address to the nation since demonstrations took place on February 20 to call for more democracy and less corruption.
"This speech included ambiguous terms that lead us to doubt the will for change and incite us to maintain the date of March 20 for new demonstrations in Morocco," the communiqué said.
"We demand a new constitution guaranteeing a genuine separation of powers and an independent justice system, as well as the removal of constitutional provisions that enshrine the sacred nature of centralised political power," the movement of February 20 added.
The demonstrations across Morocco on February 20 were largely organised by young people using Facebook to call for urgent political reforms and a curbing of the powers of the king. The Moroccan press has widely hailed the speech by Mohammed VI as "historic" in its implications, while wondering about the political future of some of the king's aides.