The wave of cyberattacks wreaking havoc across the globe is "at an unprecedented level", European Union's law enforcement agency Europol said.

The attacks will "require a complex international investigation to identify the culprits", the agency, which is working with countries and companies hit by the attacks, said in a statement on Saturday.

Dozens of countries were hit by the ransomware attack on Friday that locked up computers and held users' files for ransom at a multitude of hospitals, companies and government agencies.

It was believed to the biggest attack of its kind ever recorded.

READ MORE: Global hacking attack infects 57,000 computers

The malicious software behind the onslaught appeared to exploit a vulnerability in Microsoft Windows that was supposedly identified by the US' National Security Agency (NSA) for its own intelligence-gathering purposes and was later leaked to the internet.

Britain's National Health Service fell victim, its hospitals forced to close wards and emergency rooms and turn away patients.

Russia appeared to be the hardest hit, according to security experts, with the country's interior ministry confirming it was struck.

'Biggest outbreak in history'

All told, several cyber-security firms said they had identified the malicious software responsible for tens of thousands of attacks in 99 countries.

Mikko Hypponen, chief research officer at the Helsinki-based cyber-security company F-Secure, called the attack "the biggest ransomware outbreak in history".

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 and other files.

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Its ransom demands start at $300 and increase after two hours to $400, $500 and then $600, said Kurt Baumgartner, a security researcher at Kaspersky Lab.

Affected users can restore their files from backups, if they have them, or pay the ransom; otherwise they risk losing their data entirely.

Chris Wysopal of the software security firm Veracode said criminal organisations were probably behind the attack, given how quickly the malware spread.

"For so many organisations in the same day to be hit, this is unprecedented," he said.

The security holes it exploits were disclosed several weeks ago by TheShadowBrokers, a mysterious group that has published what it says are hacking tools used by the NSA as part of its intelligence-gathering.

Shortly after that disclosure, Microsoft announced that it had already issued software "patches" for those holes. But many companies and individuals haven't installed the fixes yet or are using older versions of Windows that Microsoft no longer supports and didn't fix.

"Microsoft stopped supporting Windows XP three years ago, hoping they would move to newer versions of the operating system," Al Jazeera's technology editor, Tarek Bazley, said.

"Many did but about 100 million computers - 10 percent of those running on Windows - still use XP, making them susceptible to an attack like this."

In addition to Russia, the biggest targets appeared to be Ukraine and India, nations where it is common to find older, unpatched versions of Windows in use, according to security firms.

READ MORE: 'Major disruption' as UK hospitals hit by cyber attack

Hospitals across Britain found themselves without access to their computers or phone systems. Many cancelled all routine procedures and asked patients not to come to the hospital unless it was an emergency. Doctors' practices and pharmacies reported similar problems.

"This attack is going to be a wake-up call for many businesses that have dragged their feet when it comes to security and updating their systems," Bazley said.

"But even if 100,000 computers are affected by this attack, it's worth remembering that this is just a tiny fraction - less than 0.0001 percent - of the billion or more computers running Windows in the world today."

British Prime Minister Theresa May said there was no evidence patient data had been compromised and added that the attack had not specifically targeted the National Health Service.

"It's an international attack and a number of countries and organisations have been affected," she said.

Ransomware attacks are on the rise around the world. In 2016, Hollywood Presbyterian Medical Center in California said it had paid a $17,000 ransom to regain control of its computers from hackers.