The letter says, "a group of Moroccans violated the Saharawi workshop, provoking a fight with the moderator, Mr. Fernando Peraita, Member of the Task Force and hindering his attempts to bring order back". Unable to continue their workshop, the Sahrawis requested the Forum's organizers send an official representative to discourage further Moroccan disruption.
As with all things, it's important to be careful about disinformation from either side, including your own. Still, I think the Sahrawis are telling the truth. Several organizations signed the letter, and ARSO linked to it. Moreover, I've heard of other Western Sahara meetings that have been threatened by Moroccan agitators.
The Moroccans at the World Social Forum had a right to say how they felt about the Western Sahara even if the similar rights of Sahrawis are denied inside the territory. Neither side, though, should be allowed to use violence or disruptive activities to silence its opposition.