The right answer is Call the emergency number (like 911 in USA).
If you have to make a single gesture it must be that one! They are well organized and relatively fast, but without warning them, there will be no help!
Things you have to say in the call:
* Describe the situation quickly.
* Give the number of victims.
* Give the apparent state of the victim.
* Specify if you did something and if you need the font).
* Give the address of the place.