The attack appeared to exploit a vulnerability purportedly identified for use by the U.S. National Security Agency and later leaked to the internet.
At least 16 organisations within the NHS, some of them responsible for several hospitals each, reported being targeted.
Sixteen National Health Service (NHS) organizations in the United Kingdom have been hit, and some of those hospitals have canceled outpatient appointments and told people to avoid emergency departments if possible.
Hospitals in areas across Britain found themselves without access to their computers or phone systems.
Former intelligence contractor Edward Snowden, who in 2013 leaked documents to journalists revealing the existence of broad USA surveillance programs, said on Twitter the NSA had built attack tools targeting US software that "now threatens the lives of hospital patients".
Hospitals affected range from London North West Healthcare Trust in the capital to University Hospitals North Midlands in central England and York Hospitals in the north. Doctors' practices and pharmacies reported similar problems.
A spokesman for the European Union's police agency, Europol, says Britain and Spain have asked for its support as they investigate the ransomware cyberattacks in those countries.
"We are not aware of any evidence that patient data has been compromised", she added.
Corporate computer systems in many other countries - including FedEx of the United States, one of the world's leading global shippers - were among those affected.
The NHS said the attack was not specifically targeted at the NHS and was affecting other organizations.
Thousands of computers around the world have been hacked and are demanding payment of $300 Bitcoin to unlock their files and systems.
It demands payment in three days or the price is doubled, and if none is received in seven days, the files will be deleted, according to the screen message.
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Cybersecurity firm Avast said it has tracked more than 75,000 attacks in 99 countries.
Some reports said Russian Federation had seen more infections than any other single country. Authorities said they were communicating with more than 100 energy, transportation, telecommunications and financial services providers about the attack.
The US Department of Homeland Security's computer emergency response team said it was aware of ransomware infections "in several countries around the world".
The Spanish government said their attacks were carried out with a version of WannaCry ransom ware that encrypted files and prompted demand for money transfers to free up the system.
Portugal Telecom was also hit by a cyber attack but no services were impacted, a spokeswoman for the company said.
The UK's state-run National Health Service declared a "major incident" after the attack, which forced some hospitals to divert ambulances and scrap operations. "It's stressful enough for someone going through recovery or treatment for cancer".
The National Cyber Security Centre (NCSC) was "working closely" with the NHS and that they will protect patient safety.
Security chiefs and ministers have repeatedly highlighted the threat to Britain's critical infrastructure and economy from cyber attacks.
The hackers, who have not come forward to claim responsibility or otherwise been identified, likely made it a "worm", or self spreading malware, by exploiting a piece of NSA code known as "Eternal Blue" that was released last month by a group known as the Shadow Brokers, researchers with several private cyber security firms said. In February 2016 a Los Angeles hospital, the Hollywood Presbyterian Medical Center, paid $17,000 in bitcoins to hackers who took control of its computers for more than a week.
He said many NHS hospitals in Britain use Windows XP software, introduced in 2001, and as government funding for the health service has been squeezed "IT budgets are often one of the first ones to be reduced".
"Affected machines have six hours to pay up and every few hours the ransom goes up", said Kurt Baumgartner, the principal security researcher at security firm Kaspersky Lab.
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