engine - 5:13 pm on May 13, 2011 (gmt 0)
I used to be involved with a local volunteer emergency service and we used to get a phone call from the designated emergency service, who would call one of our land line numbers, then, we would pass that report along to the next party, etc. It seemed to work.
Thankfully, we were never needed for anything significant, so they were mostly exercises.
Now, with many people owning mobile phones, the dissemination can come from the emergency service direct to the people, or via Twitter.