Safari malware, how to get rid?

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I clicked on a streaming video link I shouldn't have clicked on, and now have apparent malware when I open Safari, that blocks the browser from doing anything.
The pop-up window within the browser window says it's www.mac-system-alerts.com, and this:
Safari - Alert
Suspicious Activity Might Have been Detected.
Major Security Issue
To fix it please call Support for Apple
+1 800-680-4131 (Toll Free) immediately!

I tried to go to Safari/Preferences/Extensions, which Google results show as a potential solution, but for me all the options are greyed out so going that route to fix it does not work. It has completely immobilized Safari. Closing and reopening the browser, or rebooting, make no difference.

How do I get rid of it, when the options within Preferences are also immobilized?

Thank you
In case this is useful to anyone who can help me, these two screenshots show the Activity MonitorClick image for larger version

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Click image for larger version

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Les Kern's Avatar
Avast! works like a champion. And it's free.

Use a pen drive to transfer Firefox installer image or just the app installer, install, clean, done.
I will check Avast.

Re the second part of your message, I don't understand sorry...I download the Firefox installer onto a USB drive, then re-install it from the USB drive to the laptop again??? Sorry if I'm being thick...
I'd try rebooting the Mac in Safe Mode (hold the SHIFT key down while it's booting). Once in Safe Mode, try going into Safari Preferences and checking the extensions...
I just tried safe mode but unfortunately after that reboot the malware popup was still there and System Preferences still greyed out and not clickable.

I looked at Avast but it looks like it's for PCs only. There was a Mac option called TuneUpMac or something and I tried it but it wouldn't let me install Firefox to a USB drive, had to have Mac OSX.
I found this fix online.

First Force Quit Safari.

The hold down the Shift key and open Safari.

Then I clear my history.
This last one worked! Thank you all.