This is a very serious topic to me. I have been trying to brainstorm an action plan since I work for a company that could be hit by a flash mob.
1. I have taken the time to speak with employees and tell them not to intervene due to the situation becoming volatile and turning violent.
2. Once recognized, dial 911. As a liaison to law enforcement I have been trying to make contact with the local PD to come up with a game plan.
3. Utilize CCTV and try to document all involved and possibly identify a leader.
4. Remain calm.
5. Have employees to help with honest customers exit and get out of the way of harm. Use fire exits if need be.
6. Break out an M-60 and mow them down.

Oops, we best skip number 6.

7. Material items can be replaced, lives cannot so do what you can to stay safe.
Plans need to be made to cordon off and secure the area. A checkpoint and one exit should be in place in the cordon. This is what I have come up with so far.