This is a very dangerous scam. be on HIGH ALERT. Set you up to think that your computer is at risk, and try to get you to download a program that will take over your computer, providing access to user names, passwords, and banking information. STAT CLEAR!!!!!!!
Thank you all the people here that put out the warning: I got it today and before I did anything I looked up and I called Apple and got help to clean up this attack on my browser.
I received the message on my Mac, telling me to call that number, saying it's the Mac Support number and that it is a "critical security warning" and that my "IP Address seems accessed from two locations." I shut down my computer immediately, turned it back on and researched the phone number provided in the warning. Be very careful!!!