Ransomware is a form of malicious software or malware that secretly installs itself on a user's computer before blocking access to files and threatening to delete them unless a ransom is paid within a time limit.
In the UK, United Lincolnshire Hospitals NHS Trust said it was cancelling some weekend appointments because of the attack, which had affected Lincoln County Hospital, Pilgrim Hospital and Grantham Hospital. "We're taking extra precautions to protect our systems".
According to the UK media, medics across the United Kingdom reported messages flashing on their computer screens demanding money to remove the restriction.
Quite simply, ransomware locks or encrypts files or documents and demands payment to regain access. The NHS said 16 of its organizations reported they were victims.
The same malware, called WCry is also being blamed for large-scale attacks in Europe and Asia, repeatedly hitting networks in Russian Federation and China.
The NHS Digital said it was aware of the problem and would release more details soon.
Hospital trusts across England and Scotland have admitted they've been caught up in the attack, with appointments cancelled, phone lines down and ambulances divereted. But there's no evidence so far that patient data has been accessed, NHS Digital said.
It was not clear if the disruption was linked to cyber-attacks also reported Friday on several major Spanish companies including telecom giant Telefonica.
Colchester General Hospital, for example, has shut down all computer systems as a precautionary measure, Sky News reports, and issued a statement saying it was "postponing all non-urgent activity for today and we are asking people not to come to A&E".
Nationally, the NHS says it has IT specialists working on the problem.
The attack appeared to be part of a wider global hack and the NHS said it did not believe it had been specifically targeted; Spain's government said a large number of companies, including telecommunications giant Telefonica, had also suffered a ransomware attack.
A spokeswoman for West Suffolk Hospital said this evening: "We're now unaffected but we have shut down our incoming email".
Although at an early stage, the organisation, which is responsible for national digital and data initiatives across the NHS, said the malware variant thought to have infected systems was known as "Wanna Decryptor". We will continue to work with affected organisations to confirm this.
The National Cyber Security Centre, part of the GCHQ electronic intelligence agency, said it was working with police and the health system to investigate the attack.
The screenshot, posted on Twitter, shows a message stating "oops, your files have been encrypted" and demands a payment of $300 in bitcoins.