I covered something related in a comment the other day. Depending on the amount of people normal riot defence would probably be fine: rubber bullets, bean bag guns, tear gas, water cannons, pepper spray etc.

In an "oh fuck" serious invasion, providing it is just shit loads if dumb memers, a few a few white phosphorus grenades/bombs would be pretty effective. I can't speak for anyone else but if there's even the slightest chance of getting super hot, reactive, angry metal in me I'm getting the fuck away. To be fair even with actual soldiers once flesh starts melting I'm sure most people would book it with shit running down their legs.

Not just getting this from Spec Ops: The Line (awesome game btw), it's just as terrible, if not more so in real life.