Amnesty International said on Tuesday Ali Lamrabit, editor-in-chief of two satirical independent newspapers, is being held "for the legitimate and peaceful expression of his beliefs".
Lamrabit is currently serving a three-year prison sentence after expressing opinions on the monarchy and the status of the disputed province of Western Sahara.
He was convicted on charges of insulting King Muhammad VI, "undermining the monarchy", and "threatening the integrity of the national territory".
The conviction was based on several articles, cartoons and a photo-montage which had appeared in his newspapers - the French-language weekly Demain Magazine and the similarly named newspaper, Demain.
Amnesty's call comes on the eve of a court hearing on Wednesday, at which Lamrabit risks a further four months incarceration.
A Rabat court will examine an appeal, lodged by his lawyer to revoke a four-month sentence he received in November 2001.
"Ali Lamrabit should not have been imprisoned in the first place. He is a prisoner of conscience and must therefore be immediately and unconditionally released"
"Ali Lamrabet should not have been imprisoned in the first place," said Amnesty. "He is a prisoner of conscience and must therefore be immediately and unconditionally released."
Amnesty has added it urges the Moroccan authorities to either abolish or review all legislation which prescribes prison sentences for the peaceful exercise of free speech.
Lamrabit is currently on hunger strike in protest at his continued detention and has expressed his determination to continue until he is released.
He previously undertook a 47-day hunger strike in May last year in protest at his court case and imprisonment and had to be admitted to hospital.
However, the Moroccan minister of Justice, Muhammad Buzubaa, said recently that Lamrabet was in "good health and enjoys all the rights guaranteed by the law".
He described Lamrabit's case as "unfortunate" without further elaborating.
And government minister Muhammad al-Yazghi told Aljazeera on 1 January that Morocco did not arrest people for their opinions.
"On the contrary," he said, "the scope of liberties and rights has been widened, and the citizens can protest and demonstrate without fear."Ali Lamrabit founded the French-language weekly Demain Magazine in early 2001.
His previous, similarly named newspaper, Demain, was banned by the Moroccan authorities in December 2000 for "undermining the stability of the state".
This followed the publication of an article implicating the political left in a 1972 coup attempt against the late King Hasan II.
Lamrabit was also cited for "disseminating false information which undermines public order or is likely to undermine it "after publishing an article on a royal palace in Demain Magazine in October 2001.
Disputed Westerm Sahara is a
taboo subject in Morocco
The article speculated that a royal palace was to be sold to foreign investors wanting to turn it into a tourist complex.
The magazine also included an article featuring an interview with a former Moroccan political prisoner advocating the right to self-determination for Sahrawis in Western Sahara.
The monarchy and the status of Western Sahara are taboo subjects for public discussion in Morocco.
Several people, including journalists and political activists, have been imprisoned in recent months, after peacefully expressing views on these issues.
Western Sahara is a disputed territory claimed by both Morocco, which annexed it in 1975, and the Polisario Front.
Polisario calls for an independent state there and has set up a self-proclaimed government-in-exile in refugee camps, near southwestern Algeria.