Yeah I was scratching my head at this one. Cop had better have a really good reason here because otherwise, have fun getting Section 1983'd. I am not sure qualified immunity would apply against the right to peacefully assemble, unless either there was reasonably a threat of danger, or some legal authority made the assembly or its actions illegal (e.g. no one allowed on school campuses after 9pm, a citywide noise ordinance on weekdays, etc).
am not a licensed lawyer and this is neither advice nor guaranteed correct analysis... just in case.