I just got a "warning: system may have found viruses on your computer" message on safari.

I just got a "warning: system may have found viruses on your computer" message on safari that mentioned system may have found 2 malicious viruses: Rootkit.Sirefef.Spy and Trojan.FafeAV-Download. Also a phone # to call for help in removing the viruses. I have a Macbook Pro I don't know if this is for real or spam. I had to force quit safari and turn off and then back on my computer but when I clicked on safari again the message was still poping up. Does anyone know and can anyone help me? Thank you.