U.S. software firm Symantec said the majority of organisations affected were in Europe, and the attack was believed to be indiscriminate.
The majority of the attacks targeted Russia, Ukraine and Taiwan.
However, a hacker could change the code to remove the domain and try the ransomware attack again.
World Capitals - Tens of thousands of computers in nearly 150 countries were infected with ransomware demanding ransom payments during an worldwide cyberattack, which some experts believe was inspired by a National Security Agency (NSA) tool kit that was leaked previous year.
"The software has actually stopped spreading across the world", he told CNN.
The researcher, tweeting as @MalwareTechBlog, said registering a domain name used by the malware stops it from spreading, though it can not help computers already affected. The security researcher has posted an online account of finding the kill switch, which was also posted to the United Kingdom government's National Cyber Security Centre website. That affordable move redirected the attacks to MalwareTech's server, which operates as a "sinkhole" to keep malware from escaping.
But the kill switch couldn't help those already infected. "That work is still ongoing", she told BBC radio. "I am anxious about how the numbers will continue to grow when people go to work and turn on their machines on Monday morning".
As terrifying as the unprecedented global "ransomware" attack was, cybersecurity experts said it was nothing compared to what might be coming - especially if companies and governments do not make major fixes.
"Like many organizations our plant was subject to a ransomware attack affecting some of our systems on Friday evening", a spokeswoman confirmed.
But Europol Director Rob Wainwright said he feared the attack was not over and that the number of attacks would continue to grow.
The ransomware, called WannaCry, locks down files on an infected computer and asks the computer's administrator to pay in order to regain control of them.
This one worked because of a "perfect storm" of conditions, including a known and highly risky security hole in Microsoft Windows, tardy users who did not apply Microsoft's March software fix, and malware created to spread quickly once inside university, business or government networks.
Several Russian banks were also attacked by the malware, but their computer networks were not penetrated, the cybersecurity monitoring center FinCert, which is operated by Russia's central bank, reported on Saturday.
The attack held hospitals and other entities hostage by freezing computers, encrypting their data and demanding money through online bitcoin payment - $300 at first, rising to $600 before it destroys files hours later. Fortune reported Thursday that the price of bitcoin was at an all-time high.
Darien Huss, a 28-year-old research engineer who assisted the anonymous British researcher lauded a hero, said he was "still anxious for what's to come in the next few days because it really would not be so hard for the actors behind this to re-release their code without a kill switch or with a better kill switch".
The cyberattack affected 16 organizations that are part of the National Health Service on Friday, causing some surgical procedures to be canceled and ambulances diverted.
Infected computers appear to largely be out-of-date devices that organizations deemed not worth the price of upgrading or, in some cases, machines involved in manufacturing or hospital functions that proved too hard to patch without possibly disrupting crucial operations, security experts said.
A senior nurse with NHS Lanarkshire in Scotland posted a video on Twitter appealing to members of the public "to stay away from acute hospitals unless it's an absolute emergency situation" while its IT systems remain affected. "Because they could have done something ages ago to get this problem fixed, and they didn't do it".
British Interior Minister Amber Rudd announced on Saturday that 48 of 248 health services in England had been impacted by Friday's attack, but that all except six were now functioning normally.
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Grant Gowers, 50, from Clacton-on-Sea in southern England, told CNN how the ransomware attack had directly affected him. The NHS said in a statement on Saturday that there was no evidence that patient information had been compromised.
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Individuals and organisations were discouraged from paying the ransom, as it was not guaranteed that the access would be restored. He warned that more people may be hit by the virus Monday when they return to work and switch on their computers.
Currently, the White House holds daily news briefings conducted by the White House press secretary or deputy press secretary. From there, a familiar pattern ensued: a Trump tweet storm followed by more vagueness from Spicer.
It's not clear yet whether Trump has any recordings of the conversation that took place. "The president is an active president". Regarding White House audio recordings of the January 27th Trump/Comey dinner.
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