Sugar water. No outright harm, easy to clean up. Not offensive to residents … they just have to hose everything off for a couple minutes … unlike harsh chemicals, it's eco friendly to the storm drains.
But if the "protestors" don't get home, wash their clothes and themselves, the bees, ants and vermin will be relentless. If they were bused, they will invite the pests into their high-dollar buses and THEY will have to be thoroughly cleaned inside. If they car-pooled or drove themselves, same deal, except on a smaller scale. If they have set up a tent city, it will become uninhabitable in just hours.
I don't see much in the way of legal liability and the first time it happens, Antifa (et alia) will think it strange, but not see the problem immediately. On a hot summers day, though, awareness will not be long in coming.