The city "is committed to protecting individuals' right to protest," Recreation and Parks director Gregory Bayor wrote in a letter to the group dated Monday. "However, permanent camping is prohibited in public parks."
Occupy Baltimore's media team did not immediately respond to a request for comment.
The local group is part of a national movement affiliated with Occupy Wall Street that aims to highlight wealth disparity, among other issues. Los Angeles police arrested 200 members of the movement at a park Wednesday and Philadelphia authorities evicted about 100 protesters Wednesday, and later arrested about 50.
Bayor wrote that the protesters may use the park only between 8 a.m. and 11 p.m. and that "long-term camping" is not allowed. No more than 150 people may gather in the square, a small brick area near the corner of Pratt and Light Streets, he wrote.
Officials also advised the protesters that they may not stay through April, because "others have already secured permits to use McKeldin Square for holiday and other celebratory purposes." Permits are not granted for more than five days, Bayor wrote.
Recreation and Parks officials had rejected a previous application from the group.
Ryan O'Doherty, a spokesman for Mayor Stephanie Rawlings-Blake, reiterated that the city would "enforce the camping prohibition at a time of its choosing."
O'Doherty declined to say when authorities would evict the protesters. The site will likely be surrounded by revelers in a few weeks for the city's annual New Year's fireworks celebration.