Its just a fake virus. It's a web page telling you that you got a virus, and push you to call a number so they steal money from you. So don't call this number.
Now... you can't open your Safari anymore because it keep poping up right? This is because of some dumb Safari behavior. It keep reopening every web sites that you had opened before you closed it. So all what you got to do to get out of this trap is to close Safari, then PRESS & HOLD SHIFT KEY on your keyboard while you click on Safari to open it again. It will tell your Safari to NOT open the last opened pages, and voila... you are good to go.
You can install an antivirus if you want, but in this particular case, it's not necessary.
Just got a call from someone in my family with the same problem so i thought about sharing the experience...
Have a nice day
Chris