Morocco's youth-based 20 February Movement on Saturday rejected constitutional reforms proposed by King Mohammed VI, calling for nationwide protests.
"The plan as proposed by the king yesterday (Friday) does not respond to our demands for a true separation of powers. We will protest peacefully on Sunday against this plan," a member of the movement's Rabat section told AFP.
The king outlined curbs to his wide political powers in an address to the nation, pledging to build a constitutional monarchy with a democratic parliament.
The proposals, to be put to a referendum 1 July, devolve many of the king's powers to the prime minister and parliament. They come in the wake of nationwide pro-reform demonstrations that started 20 February, inspired by popular uprisings sweeping the Arab world.
The 47-year-old monarch, who in 1999 took over the Arab world's longest-serving dynasty, currently monopolises power in the Muslim North African country. He is also its top religious authority, as Commander of the Faithful.
Under the new draft constitution, the king will retain this religious role and remain head of state. His person would be "inviolable," while the "free exercise of religion" would be guaranteed by the monarch, the king said in his speech. The king would also remain head of the army and still appoint ambassadors and diplomats. He would also retain the right to name top officials in "strategic" administrations.
Many political players welcomed Mohammed VI's proposals. "Compared to the current constitution, this plan is an important advance," Saad Eddine Othmani, an opposition lawmaker, told AFP. "Everything the king promised in his speech of 9 March has been retained."
The 9 March speech was the king's first since the uprisings that toppled autocratic rulers in Tunisia and Egypt and came less than a month after protests erupted in Morocco for social justice and limits on royal power.
"But is this advance enough? That is what we will discuss today (within the Justice and Development Party)," an Islamist opposition party, Othmani said.
The head of the government coalition partner, the Party of Progress and Socialism, Nabil Benabdallah, also praised the speech.
"Morocco is entering a new constitutional phase. This plan will allow the construction of a modern democratic state," he said.
But the 20 February Movement, demanding deep political reforms, called for more peaceful protest, including in Rabat, Casablanca, Tangiers, Marrakesh and Fez, according to the movement's Facebook page, which has more than 60,000 members.
The youth-led group has brought thousands of people to the streets in unprecedented calls for change and an end to corruption.