Annoying scam
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****WARNING Mac Virus Alert!****
You might be infected with Adware / Spyware Virus etc etc
Contact Mac Support for Immediate Help
(0800)-011-9608 (Toll Free)
And so it went on, telling me not to shut down or restart my computer etc. etc.
I quit Safari, but when I opened it up again I got the same message, as Safari opened again from the page that I was on when I got the original message. So I quit it again and Googled the telephone number using Firefox, where I was able to see with some relief, that it was a scam activated by the web page I was on, and gave the advice to restart Safari with the Shift key pressed down so that it opens with none of the previous tabs open. I did, and all was well again.