Break into any standard residential house, and head to the bathroom. It will almost always have a toilet brush. Take it, and escape the house.
Now head to somewhere populated, such as a carpark, childrens park or even a graveyard. Equip the "weapon", making sure you don't have it holstered (eww) and go to town on the peeps.
Of course, it helps if you've upgraded your melee attack for this one, but as long as you have enough people, time isn't an issue.