Martial Law, the door to door seizures of weapons (Katrina) The use of Military Force to maintain order (Los Angeles Riots).
These are recent examples, but basically gatherings of more than 3 people would become illegal, Curfews would be inforced (many curfews are already on the books just waiting). Movement of people would be enacted, your house is no longer safe for you to live(we shut off your sewer), so we are taking you to a safer camp(it has no sewer).
REMEMBER UN BLUE MAKE A NICE TARGET PICTURE.