But the National Health Service in the United Kingdom and global firms such as Fedex also reported they had come under assault Friday.
"At this stage", the NHS said, "we do not have any evidence that patient data has been accessed". Several were forced to cancel or delay treatment for patients.
Some ransomware that encrypts files ups the stakes after a few days, demanding more money and threatening to delete files altogether.
In Britain, the attack disrupted care at National Health Service facilities, forcing ambulances to divert and hospitals to postpone operations. But it appears to be "low-level" stuff, Eisen said Saturday, given the amount of ransom demanded - $300 at first, rising to $600 before it destroys files hours later.
Edward Snowden, the former NSA contractor who leaked evidence of the agency's data collection program in 2013, has spoken out on Twitter to criticize the NSA for building this "dangerous attack tool".
"It's an global attack and a number of countries and organizations have been affected", she said. But the ministry's website still carried a banner on Saturday afternoon saying that technical work was continuing. "Our teams are working to resolve the issue", said the spokesman.
Many of the British hospitals that were victims to the attack were using the outdated software, experts said.
"The recent attack is at an unprecedented level and will require a complex worldwide investigation to identify the culprits", said Europol, Europe's policing agency.
"This is not targeted at the NHS", British Prime Minister Theresa May was quoted as saying in BBC.
All told, several cyber security firms said they had identified the malicious software, which so far has been responsible for tens of thousands of attacks, in more than 60 countries. CNN has not independently confirmed that number.
Yet earlier on Saturday the Home Secretary said: "They [NHS trusts and health boards] have been asked to move from Windows XP, the Secretary of State for Health has been very clear in that direction".
"There will be lessons to learn from what appears to be the biggest criminal cyber attack in history", she said.
NHS Digital, which runs the health service's IT systems, has said 16 hospital trusts are among the organisations affected, of which five "major trusts" have been impacted seriously, sources told Sky News.
The East and North Hertfordshire NHS Trust was "experiencing significant problems with our telephone network", it said in an online statement.
Hospitals across Britain found themselves without access to their computers or phone systems.
At St. Bartholomew's Hospital in central London, Sean, who did not want to give his last name, said he noticed problems with the network as soon as he arrived.
It spreads from computer to computer as it finds exposed targets.
They said cyber incidents represent a growing threat to their economies and should be tackled as a priority.
Managers at many companies and other organizations have not taken steps to put proper cybersecurity systems in place despite talking about their importance, Gazeley said.
"The large-scale cyberattack on our NHS today is a huge wake-up call", said Jamie Graves, CEO of cybersecurity company ZoneFox.
If your computer has been affected, there's no guarantee that paying the ransom will restore it, Gazeley said. "The only sensible way to tackle it is to "pull the plug" so that it can't spread any more until you can isolate the affected machines and work out a remediation plan".
In December it was reported almost all NHS trusts were using an obsolete version of Windows that Microsoft had stopped providing security updates for in April 2014.
"It is a disgusting lesson about why using supported software, and keeping that software updated, is so important", he said.
Pete Turner, from cyber security firm Avast, said: "It's critical that organisations and employees, particularly those in our most critical sectors like healthcare, start to think pro-actively about how to protect themselves from ransomware".
"Our society increasingly relies on interconnected systems to deliver key services such as health", he said.