The unprecedented global cyberattack has hit more than 200,000 victims in scores of countries, Europol said Sunday, warning that the situation could escalate when people return to work.
An international manhunt was well under way for the plotters behind what was being described as the world's biggest-ever computer ransom assault.
The indiscriminate attack, which began Friday, struck banks, hospitals and government agencies in more than 150 countries, exploiting known vulnerabilities in old Microsoft computer operating systems.
US package delivery giant FedEx, European car factories, Spanish telecoms giant Telefonica, Britain's health service and Germany's Deutsche Bahn rail network were among those hit.
Europol executive director Rob Wainwright said the situation could worsen on Monday as workers return to their offices after the weekend and log on.
"We've never seen anything like this," the head of the European Union's policing agency told Britain's ITV television, calling its reach "unprecedented".
"The latest count is over 200,000 victims in at least 150 countries. Many of those victims will be businesses, including large corporations.
"We're in the face of an escalating threat.
"I'm worried about how the numbers will continue to grow when people go to work and turn on their machines on Monday."
- 'Ooops' message, $300 ransom -
Images appear on victims' screens demanding payment of $300 (275 euros) in the virtual currency Bitcoin, saying: "Ooops, your files have been encrypted!"
Payment is demanded within three days or the price is doubled, and if none is received within seven days the locked files will be deleted, according to the screen message.
Experts and governments alike warn against ceding to the demands and Wainwright said few victims so far have been paying up.
"Paying the ransom does not guarantee the encrypted files will be released," the US Department of Homeland Security's computer emergency response team said.
"It only guarantees that the malicious actors receive the victim's money, and in some cases, their banking information."
The culprits used a digital code believed to have been developed by the US National Security Agency -- and subsequently leaked as part of a document dump, according to researchers at the Moscow-based computer security firm Kaspersky Lab.
A hacking group called Shadow Brokers released the malware in April, claiming to have discovered the flaw from the NSA, Kaspersky said.
Europol's Wainwright said the attack was unique because the ransomware was combined with a "worm" -- meaning the infection of one computer could automatically infect an entire network.
Microsoft said the situation was "painful" and that it was taking "all possible actions to protect our customers".
It issued guidance for people to protect their systems, while taking the highly unusual step of reissuing security patches first made available in March for Windows XP and other older versions of its operating system.
- Banks, trains and automobiles -
US software firm Symantec said the majority of organisations affected were in Europe.
The companies and government agencies targeted were diverse.
Europol's Wainwright said few banks in Europe had been affected, having learned through the "painful experience of being the number one target of cyber crime" the value of having the latest cyber security in place.
Russia's interior ministry said some of its computers had been hit, while the country's banking system was also attacked, although no problems were detected, as was the railway system.
French carmaker Renault was forced to stop production at sites in France, Slovenia and Romania, while FedEx said it was "implementing remediation steps as quickly as possible".
Germany's rail operator Deutsche Bahn said its station display panels were affected. Universities in China, Italy and Greece were also hit.
On Saturday, a cyber security researcher tweeting as MalwareTechBlog, said he had accidentally discovered a "kill switch" that could prevent the spread of the ransomware.
The anonymous researcher said registering a domain name used by the malware stops it from spreading, though it cannot help computers already affected.
On Sunday, the researcher warned that hackers could upgrade the virus to remove the kill switch.
"Version 1 of WannaCrypt was stoppable but version 2.0 will likely remove the flaw. You're only safe if you patch ASAP," he tweeted.
Meanwhile G7 finance ministers meeting in Italy vowed to unite against cyber crime, as it represented a growing threat to their economies and should be tackled as a priority.
The danger will be discussed at the G7 leaders' summit next month.