Tens of thousands of computers in 99 countries have been infected by a ransomware virus which extorts users by blocking Windows files and demanding payment to restore access.
Security researchers at Kapersky Lab said that Russia was the worst hit.
Other countries that were in the top 20 worst affected included Ukraine, India, Taiwan, China, Romania, Egypt, Iran, Brazil, Spain and Italy.
Around 1,000 computers at the Russian Interior Ministry have been affected by the huge cyber attack that hit countries including Britain and Spain today, a spokeswoman for the ministry told Interfax. The ministry reportedly did not lose any information in these attacks.
Tech firm F-Secure said it has received reports from more than 60 countries.
Mikko Hypponen, its chief research officer, called it “the biggest ransomware outbreak in history”.
And the Director of Global Research and Analysis Team at Kaspersky Lab said the number of cyber attacks is still growing.
Researchers believe a criminal organisation is behind this, given its sophistication.
Many hospitals and GP surgeries have been forced to shut down their entire IT systems, after ransomware notes from hackers appeared on their computers, threatening to delete all their files in seven days unless $300 in bitcoin currency is paid.