United Kingdom publication The Guardian reported that as many as 16 National Health Service trusts, some of which oversee several hospitals, were affected in the attack and that hospital staff were unable to access patient records.
The ransomware - dubbed Wanna Cry - demanded payments between $300 (around Rs 19,000) and $600 (around Rs 39,000) in bitcoin to unlock data on a single system, news agency Reuters reported.
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.
"At this stage we do not have any evidence that patient data has been accessed", NHS Digital, a division of NHS England, said in a statement.
Details of patient records and appointment schedules, as well as internal phone lines and emails, have all been rendered inaccessible. When asked to confirm that Wana Decryptor has struck in the US, and at what scale, Acting Deputy Press Secretary Scott McConnell did not provide specifics.
According to Ryan Kalember, senior Vice President of cyber security strategy at the cybersecurity firm Proofpoint, a "ransomware worm" using the essentially unaltered NSA code is spreading across corporate networks in at least 74 countries, with European and Asian countries among the hardest hit.
Computers in regions across the globe have been under attack today, including Telefonica (O2 in the UK), with at least 45,000 computers compromised in Russia, Ukraine, India, and Taiwan alone.
Pictures posted on social media showed screens of NHS computers with images demanding payment of 300 USA dollars worth of the online currency Bitcoin, saying: "Ooops, your files have been encrypted!"
No patient data was believed to have been accessed during the attack, and Britain's National Cyber Security Centre and National Crime Agency have said that they are now investigating the issue.
A Blackpool CCG spokesman confirmed to the BBC that the attack had shut down the majority of their systems, affecting GPs, hospitals and walk-in centres. A spokesperson for the Trust said, whilst they had detected the malware, it had "not had a significant impact on our organisation".
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A massive ransomware attack is now under way.
Russian media also said that the Investigative Committee, the nation's top criminal investigation agency, also has been targeted.
Some of the companies that were targeted in the worldwide cyberattack included global shipper FedEx Corp, Spanish telecommunications company Telefonica and French aircraft manufacturer Airbus.
NHS services across England have been hit by either IT failure or "ransomware" - where a message comes up demanding money for the recovery of files or else they will be deleted. Many canceled all routine procedures and asked patients not to come to the hospital unless it was an emergency.
NHS Digital said it was working with the government's National Cyber Security Centre, the Department of Health and NHS England to help the organizations affected "manage the incident swiftly and decisively".
Barts Health NHS Trust in London said on its website it was "experiencing a major IT disruption and there are delays at all of our hospitals".
No group has been identified as responsible for the attack so far.
"Looking at the trends, it was going to happen", he said.
"Otherwise, such an attack can be technically un-investigable", the statement said. Security researchers said they observed some victims paying via the digital currency bitcoin, though they did not know what percent had given in to the extortionists.
He said most likely it occurred this time because some of the hospitals and other organizations affected may not have applied a patch that Microsoft released or they are using outdated operating systems no longer supported by the software giant.