In the United Kingdom, some 48 public health organizations were affected by the attack, which left doctors and hospitals incapable of accessing patient data, which resulted in the cancellation of a large number of appointments.
The worldwide effort to extort cash from computer users is so unprecedented in its nature - the first widely successful example of ransomware that self-replicates like a virus - that Microsoft quickly changed its policy, announcing free security patches to fix this vulnerability in the older Windows systems still used by millions of individuals and smaller businesses. He said that mobile communications haven't been affected.
The hackers used a technique described in leaked files from the US National Security Agency to spread the ransomware by exploiting weaknesses in Windows XP.
"WannaCry" encrypts files with the following extensions, appending.WCRY to the end of the file name like.lay6, .sqlite3, .sqlitedb, .accdb, .java and.docx among others.
"No virus infection happened", it said. Two security firms - Kaspersky Lab and Avast - said they identified the malicious software in more than 70 countries.
Meanwhile, patients have told Sky News how there was chaos and pandemonium in hospitals as computers shut down one-by-one and doctors lost track of patient notes, blood tests and drugs.
Barts NHS Trust, which runs these hospitals, said patients with planned appointments on Monday will be contacted if they can not be seen under its reduced service.
Telecoms firm Telefonica was one of those reporting problems, along with courier firm FedEx.
Earlier, Rudd said the government did not know who was behind the attack, which also hit some companies. Hospitals, with their often outdated IT systems and trove of confidential patient data, are a particularly tempting target.
Speaking after chairing a meeting of the crisis response committee, or COBRA, Rudd said 48 of 248 health service trusts in England had been impacted by the attack, but that all except six were now functioning normally.
"It's important to understand that cyberattacks can be different from other forms of crime in that their sometimes highly technical and anonymous nature means it can take some time to understand how it worked, who was behind it and what the impact is", he told the BBC.
"If you have anything to patch, patch it", the researcher said in a blog post.
Pyotr Lidov, a spokesman for Megafon, said Friday's attacks froze computers in company's offices across Russian Federation.
Forcepoint originally said in a statement that the attack had "global scope", affecting organisations in Australia, Belgium, France, Germany, Italy and Mexico. It initially said 16 NHS organizations had reported being hit, and more reports came in as the day went on.
"Unlike most other attacks, this malware is spreading primarily by direct infection from machine to machine on local networks, rather than purely by email", Lance Cottrell, chief scientist at the U.S. technology group Ntrepid. The National Center for the Protection of Critical Infrastructure says Friday it was communicating with more than 100 providers of energy, transportation, telecommunications and financial services about the attack. The ransomware encrypted data on the computers, demanding payments of $300 to $600 in Bitcoins to restore access.
A spokesman for NHS Digital, which manages health service cyber security, said: "At this stage, we do not have any evidence that patient data has been accessed".
Security experts said the attack appeared to be caused by a self-replicating piece of software that enters companies and organisations when employees click on email attachments, then spreads quickly internally from computer to computer when employees share documents.