They can defend the premises by using non-lethal force. Rubber bullets, tear gas, water cannons. Even if 20,000 showed up they could handle it without having to resort to lethal force. If things get out of hand they'd just need to fire a couple of live rounds, maybe kill one person, and the rest will scatter. This isn't like a popular revolt or riot, the people who show up are doing it for a joke and they won't have the anger or courage necessary to stand and fight once they run into opposition