Computers in thousands of locations have apparently been locked by a program that demands $300 in Bitcoin.
There have been reports of infections in more than 70 countries, including the UK, US, China, Russia, Spain, Italy and Taiwan.
Many security researchers are linking the incidents together.
The UK's National Health Service (NHS) was also hit by a ransomware outbreak on the same day and screenshots of the WannaCry program were shared by NHS staff.
One cyber-security researcher tweeted
that he had detected many thousands of cases of the ransomware - known
as WannaCry and variants of that name - around the world.
"This is huge," said Jakub Kroustek at Avast.
Security firm Kaspersky Lab has recorded more than 45,000 attacks in 74 countries in the past 10 hours.
Seventy-four countries around the globe have been affected, with the
number of victims still growing, according to Kaspersky Lab. According
to Avast, over 57,000 attacks have been detected worldwide, the company
said, adding that it "quickly escalated into a massive spreading."
Several experts monitoring the situation have linked the
infections to vulnerabilities released by a group known as The Shadow
Brokers, which recently claimed to have dumped hacking tools stolen from the US National Security Agency (NSA).
A patch for the vulnerability was released by Microsoft in March, but many systems may not have had the update installed.
security researchers have pointed out that the infections seem to be
deployed via a worm - a program that spreads by itself between
'Turn off PCs'
A number of Spanish firms were among the apparent victims elsewhere in Europe.
Telecoms giant Telefonica said in a statement that it was aware of a "cybersecurity incident" but that clients and services had not been affected.
Power firm Iberdrola and utility provider Gas Natural were also reported to have suffered from the outbreak.
There were reports that staff at the firms were told to turn off their computers.
At least one local authority in Sweden also appears to have been a victim.
have around 70 computers that have had a dangerous code installed," the
mayor of Timra, a town to the north of Stockholm, told the Reuters news
Explaining the global ransomware outbreak
many other malicious programs, this one has the ability to move around a
network by itself. Most others rely on humans to spread by tricking
them into clicking on an attachment harboring the attack code.
contrast, once WannaCry is inside an organisation it will hunt down
vulnerable machines and infect them too. This perhaps explains why its
impact is so public - because large numbers of machines at each victim
organisation are being compromised.
Screenshots of WannaCry with text in Spanish were also shared online.
In Italy, one user shared images appearing to show a university computer lab with machines locked by the same program.
Bitcoin wallets seemingly associated with the ransomware were reported to have started filling up with cash.
firm that confirmed it had been caught out was delivery company FedEx,
though it did not clarify in which territories it had been hit.
many other companies, FedEx is experiencing interference with some of
our Windows-based systems caused by malware," it said in a statement.
"We are implementing remediation steps as quickly as possible."
Portugal Telecom also confirmed it was struck: "But none of our services were affected," a spokeswoman told Reuters.
a spokesman for Megafon, the second largest mobile phone network in
Russia confirmed some of its computers had been infected also.
is a major cyber attack, impacting organisations across Europe at a
scale I've never seen before," said security architect Kevin Beaumont.
According to security firm Check Point, the version of the ransomware that appeared today is a new variant.
"Even so, it's spreading fast," said Aatish Pattni, head of threat prevention for northern Europe.
Curled from: BBC