Pop up Virus scam?

I was on my safari on my MacBook Air 13' and a new web page opened saying "your system is infected with 3 viruses." And telling me that I should scan now, also they had a countdown at the bottom of the page. I was in a bit of a panic so I deleted the tab that had the 'virus check' and that's all I'm aware of. Is this pop-up real, should I be worried that I have a virus on my Mac?