LONDON >> A global “ransomware” attack, unprecedented in scale, had technicians scrambling to restore Britain’s crippled hospital network Saturday and secure the computers that run factories, banks, government agencies and transport systems in many other nations.

The worldwide cyberextortion attack is so unprecedented, in fact, that Microsoft quickly changed its policy, announcing security fixes available for free for the older Windows systems still used by millions of individuals and smaller businesses.

After an emergency government meeting Saturday in London, Britain’s home secretary said one in five of 248 National Health Service groups had been hit. The onslaught forced hospitals to cancel or delay treatments for thousands of patients, even some with serious aliments like cancer.

Home Secretary Amber Rudd said 48 NHS trusts were affected and all but six were now back to normal. The U.K.’s National Cyber Security Center said it is “working round the clock” to restore vital health services.

Security officials in Britain urged organizations to protect themselves by updating their security software fixes, running anti-virus software and backing up data elsewhere.

Who perpetrated this wave of attacks remains unknown. Two security firms — Kaspersky Lab and Avast — said they identified the malicious software in more than 70 countries. Both said Russia was hit hardest.

“This is obviously by far the worst ransomware outbreak we’ve seen in, I think, forever,” said Lawrence Abrams, a New York-based malware expert who runs BleepingComputer.com.

And all this may be just a taste of what’s coming, a leading cyber security expert warned.

Computer users worldwide — and everyone else who depends on them — should assume that the next big “ransomware” attack has already been launched, and just hasn’t manifested itself yet, Ori Eisen, who founded the Trusona cybersecurity firm in Scottsdale, Arizona, told The Associated Press.

The attack held hospitals and other entities hostage by freezing computers, encrypting data and demanding money through online bitcoin payments. 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.

He said the same thing could be done to crucial infrastructure, like nuclear power plants, dams or railway systems.

“This is child’s play, what happened. This is not the serious stuff yet. What if the same thing happened to 10 nuclear power plants, and they would shut down all the electricity to the grid? What if the same exact thing happened to a water dam or to a bridge?” he asked.

“Today, it happened to 10,000 computers,” Eisen said. “There’s no barrier to do it tomorrow to 100 million computers.”

This is already believed to be the biggest online extortion attack ever recorded, disrupting services in nations as diverse as the U.S., Russia, Ukraine, Spain and India. Europol, the European Union’s police agency, said the onslaught was at “an unprecedented level and will require a complex international investigation to identify the culprits.”

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. The NSA tools were stolen by hackers and dumped on the internet.

A young cybersecurity researcher has been credited with helping to halt the ransomware’s spread by accidentally activating a so-called “kill switch” in the malicious software.

The 22-year-old Britain-based researcher, identified online only as MalwareTech, explained Saturday how he inadvertently discovered Friday that the software’s spread could be stopped by registering a garbled domain name. His $11 purchase of the name may have saved governments and companies around the world millions, slowing its spread before U.S.-based computers were more widely infected.

In the U.S., FedEx Corp. reported that its Windows computers were “experiencing interference” from malware, but wouldn’t say if it had been hit by ransomware. Other impacts in the U.S. were not readily apparent on Saturday.

The kill switch couldn’t help those already infected, however. Short of paying, options for these individuals and companies are usually limited to recovering data files from a backup, if available, or living without them.

Security experts said it appeared to be caused by a self-replicating piece of software that enters companies when employees click on email attachments, then spreads quickly as employees share documents.

The security holes it exploits were disclosed weeks ago by TheShadowBrokers, a mysterious group that published what it said are hacking tools used by the NSA. Microsoft swiftly announced that it had already issued software “patches” to fix those holes, but many users haven’t yet installed updates or still use older versions of Windows.

Before Friday’s attack, Microsoft had made fixes for older systems, such as 2001’s Windows XP, available only to mostly larger organizations that paid extra for extended technical support. Microsoft says now it will make the fixes free for everyone.

Krishna Chinthapalli, a doctor at Britain’s National Hospital for Neurology & Neurosurgery who wrote a paper on cybersecurity for the British Medical Journal, said many British hospitals still use Windows XP software, introduced in 2001.

In Russia, government agencies insisted that all attacks had been resolved. Russian Interior Ministry, which runs the national police, said the problem had been “localized” with no information compromised. Russia’s health ministry said its attacks were “effectively repelled.”

“When we say that the health ministry was attacked, you should understand that it wasn’t the main server, it was local computers … actually nothing serious or deadly happened yet,” German Klimenko, a presidential adviser, said on Russian state television.

Russian cellular phone operators Megafon and MTS were hit. The national railway said it was attacked but operations were unaffected. Russia’s central bank said Saturday that no incidents were “compromising the data resources” of Russian banks.

Germany’s national railway said Saturday departure and arrival display screens at its train stations were affected, but there was no impact on actual train services. Deutsche Bahn said it deployed extra staff to help customers.

French carmaker Renault’s assembly plant in Slovenia halted production after it was targeted. Radio Slovenia said Saturday the Revoz factory in the southeastern town of Novo Mesto stopped working Friday evening to stop the malware from spreading.

Elsewhere in Europe, the attack hit Spain’s Telefonica, a global broadband and telecommunications company, and knocked ticketing offline for Norway’s IF Odd, a 132-year-old soccer club.