Multinational shipping companies, banks, advertising firms and airports have all ground to a halt, after a major cyber attack affected operations across the world.
Dubbed "GoldenEye" or "Petya", Ukraine and Russia were the first to report the attack, followed by Romania, the Netherlands, Norway, Britain and the United States.
Cyber security experts have warned the rest of world to be on the look-out.
Yaroslav Trakalo, from Ukraine's National Police, says authorities are working to stop the virus' spread.
"As of now, the cyber police department of the National Police of Ukraine has received around 200 requests about interference in the work of computer networks. These have been requests from both state and private institutions. These requests have been received in both the Kiev and regional headquarters of cyber police. They are all being registered. Cyber police representatives are heading to the institutions, they are working to prevent dissemination of this harmful software and elimination of its consequences."
Russian oil producer Rosneft, one of the world's biggest crude producers, says it has switched to back-up systems after the hack.
Ukraine's state power company and Kiev's main airport have also experienced major disruptions, while the Chernobyl nuclear power plant's radiation monitoring system was taken offline.
Danish shipping company Maersk, American pharmaceutical company Merck & Co. and British advertising agency WPP are among those to have reported problems with their computer networks.
Russian cybersecurity firm Kaspersky Lab is suggesting it may be a new type of malware - malicious software.
Others suggest hackers may have exploited code called 'Eternal Blue', developed by the US National Security Agency, which was leaked and then used in the WannaCry ransomware attack in May.
WannaCry affected more than 150 countries, locking users' files and demanding payment to regain access to them.
In a statement posted online, Ukraine's Prime Minister has called the hack "unprecedented".
Spokesman for Ukraine's Interior Ministry, Artem Shevchenko, has told Al Jazeera he believes Russia is behind the attack.
"They attacked the state sector and the corporate sector, post offices, banks, transport, infrastructure, railway station, the main office, and other facilities. This is another example, from our point of view, of using cyber attacks in (Russia's) hybrid war against our country. At this time I can say that a criminal investigation has been initiated. The matter is that, for us, the Russian Federation is the first suspicious state because now we are involved in the war against this aggressive Russian Federation state."
But cyber-security Professor Alan Woodward, from the University of Surrey, says it's more likely to be criminals.
"The organised crime gangs are big, very big, and they make hundreds of millions out of this. I've seen one ransomware campaign where I think they got to £350 million. It's not to say that some smaller, rogue states - it's something that North Korea has done in the past, they have committed cyber-crime to raise foreign currency. But, actually we don't know, there was a suggestion that it was Russia behind this because Ukraine was hit first but that was a bit of a knee-jerk reaction. There's no evidence, there's certainly no electronic evidence I've seen, and in fact when then Russian banks started getting hit that's when ... that's the problem with these cyber weapons - if you launch them they can come back and hit you so a lot of nation states will not want to launch them in the first place. So I suspect it's just greedy criminals."
