updated 4/8/2013 4:46:21 PM ET 2013-04-08T20:46:21

Computer-security firm Sophos has received a number of reports from German Internet users who've had their computers taken hostage by ransomware that displays graphic images as purported evidence that the victim's computer has been used to access child pornography.

The bogus warning message, which claims to come from the Bundeskriminalamt, the German equivalent of the FBI, doesn't just demand that victims pay a fine.

 It also shows real pornographic images of children, complete with their alleged birth dates and nationalities, and says that the images had been accessed from the victims' machine.

When accessed from a British Internet Protocol address, the same message appears, but this time it claims to come from London's Metropolitan Police.

"Your Personal Computer has been blocked," the British version of the message reads. "The work of your computer has been suspended on the grounds of unauthorized cyberactivity.

"All the illegal actions that you performed on this computer were recorded and classified in the Police Database. This also includes photos and videos that were taken by your Web camera for further identification. You've been charged with viewing pornography that involves minors."

To unnerve victims even further, the malicious software also shows victims their own IP addresses and Internet service providers, and displays what their webcams are currently viewing.

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Sophos published censored versions of the disturbing ransomware images and said it had informed authorities including Britain's Internet Watch Foundation.

This isn't the first time criminals have used messages that purport to come from law-enforcement agencies in order to extort cash from unsuspecting Internet users, and it won't be the last. In fact, there's a whole genre of ransomware called " police Trojans " that function this way.

But there are a few things everyone should know to protect themselves from such attacks.

If you find yourself faced with a threat or warning that freezes your computer and demands cash, don't pay it. Law enforcement does not levy fines via electronic extortion. It's in your best interest to contact the real police.

To free up your data, try starting your computer from a Windows installation disk or a Linux live CD and see if you can view the files.

To avoid a situation like this altogether, users should use common sense online by treating all links with some skepticism. Hover your mouse over any in-text hyperlink to see where it really leads. If it looks suspicious, err on the side of caution and don't click through.

Last but not least, install robust anti-virus software that will protect your Web browser from drive-by downloads and screen email for malicious attachments.

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