The store should have a way to lock the front door when they suspect something like this is going to happen lock them all in then call the police or better still have the store owner armed with a shot gun to hold them till police arrive.
...which would easily be defeated by propping open the doors.
...have the store owner armed with a shot gun to hold them...
...inviting lawsuits against the store owner/shopkeeper for unlawful arrest/detainment by those in the store not actually doing anything wrong: "I didn't know ANY of those OTHER people, officer."