This screen was photographed at Britain's National Health Service.
Hospitals, major companies and government offices were among those that were badly affected.
But gas and electricity companies use an "air gap network" that is completely independent from the web and therefore less vulnerable to attack. The MalwareTech tracker detected over 100,000 infected systems over the past 24 hours.
The assault is part of an attack that has affected organizations in more than 70 countries, including the United States, China, Russia and Spain, disrupting power and telephone companies.
The U.S. government on Saturday issued a technical alert with advice on how to protect against the attacks, asking victims to report attacks to the Federal Bureau of Investigation or Department of Homeland Security.
The ransomware, called WannaCrypt or WannaCry, locks down all the files on an infected computer and asks the computer's administrator to pay to regain control of them. The malware spreads through email. Ransomware is a type of malware that essentially takes over a computer and prevents users from accessing data on it until a ransom is paid, he said.
With its long history, it's a virtual certainty that a cyber thief is now designing the next ransomware software.
Investigators are working to track down those responsible for the ransomware used on Friday, known as Wanna Decryptor or WannaCry.
Oliver Gower, of the UK's National Crime Agency, said: "Cyber criminals may believe they are anonymous but we will use all the tools at our disposal to bring them to justice".
The DHS has a cadre of cyber security professionals that can provide expertise and support to critical infrastructure entities, it said in a statement.
In Russia, where a wide array of systems came under attack, officials said services had been restored or the virus contained.
Darien Huss, a 28-year-old research engineer who helped stop the malware's spread, 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".
In England the critically ill were diverted to unaffected hospitals as computer systems failed in A&E units and doctors were locked out of test results, x-rays and patient records.
The revelation came as experts said they are working round-the-clock to restore the IT systems of NHS after they were hit by a large-scale cyber-hack by an worldwide criminal gang that wreaked havoc around the world including India.
And two years earlier Israel hackers were said to have paralysed the Syrian air defence system, allowing them to bomb President Assad's nuclear facilities.
"The global reach is unprecedented".
"Despite warnings, (NSA) built risky attack tools that could target Western software", Snowden said.
But we have seen examples in the past that remind us that even the most fortified barriers can be penetrated.
The move gave companies and institutions time to patch their systems to avoid infection before the attackers could change the code and get the ransomware going again.
On Friday a massive cyberattack spread across 74 countries, infiltrating global companies like FedEx and Nissan, telecommunication networks and most notably the UK's National Health Service.
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