Ransomware is a form of malicious software that locks up the files on your computer, encrypts them, and demands that you pay to get your files back.
The hunt was on for the culprits behind the assault, which was being described as the biggest cyber ransom attack ever.
The ransomware appeared to exploit a vulnerability in Microsoft Windows that was purportedly identified by the U.S. National Security Agency for its own intelligence-gathering purposes.
Hospitals and doctors' surgeries in parts of England were forced to turn away patients and cancel appointments.
By Friday evening, the ransomware had spread to the United States and South America, though Europe and Russian Federation remained the hardest hit, according to security researchers Malware Hunter Team.
Those affected see a message on their computer screens demanding payment in the digital currency bitcoin to restore access.
Markus Jakobsson, chief scientist with security firm Agari, said that the attack was "scattershot" rather than targeted.
The researcher, tweeting as @MalwareTechBlog, said the discovery was accidental, but that registering a domain name used by the malware stops it from spreading.
Twelve Scottish health boards, including the ambulance service, have been affected by a major cyber attack which has hit organisations around the world.
The attacks apparently exploited a flaw exposed in documents leaked from the US National Security Agency (NSA).
Microsoft released a patch for the issue in March but computers that have not been updated are still vulnerable to the attack.
Microsoft said on Friday it would roll out the update to users of older operating systems "that no longer receive mainstream support", such Windows XP, Windows 8 and Windows Server 2003.
English hospitals and clinical commissioning groups (CCGs) in London, Blackpool, Hertfordshire and Derbyshire were among those to report problems.
Patrick Ward, a 47-year-old sales director, said his heart operation, which was scheduled for Friday, was cancelled at St. Bartholomew's Hospital in London. Individual NHS trusts have asked registered patients not to attend unless it is urgent.
"We are aware that a number of NHS organisations have reported that they have suffered from a ransomware attack". "It's stressful enough for someone going through recovery or treatment for cancer".
British Prime Minister Theresa May has confirmed that the massive ongoing cyberattack that caused chaos in the NHS on Friday is part of a broader worldwide attack.
"All necessary steps are being taken to ensure that the cause and nature of this attack is identified".
Friday's attack largely hit businesses and large organizations: United Kingdom hospitals, a Spanish telecom, FedEx, the Russian Interior Ministry.
The ransomware attack disrupted business at organizations and agencies across the world, CNN reported. If the ransom isn't paid, the data is often lost forever.
The cyberattack was first reported from Sweden, Britain and France.
He said: "It's unbelievably disgusting and I've got nothing but contempt for those people that have done it, and I'm sure all of you would share that". "I did not expect an attack on this scale".