A massive cyber-attack using tools believed to have been stolen from the US National Security Agency has struck organisations around the world.
Cyber-security firm Avast said it had seen 75,000 cases of the ransom ware – known as WannaCry and variants of that name around the world.
There are reports of infections in 99 countries, including Russia and China.
Among the worst hit was the National Health Service (NHS) in England and Scotland.
Reports say about 40 NHS organisations and some medical practices were hit, with operations and appointments cancelled.
The malware spread quickly on Friday, with medical staff in the UK reportedly seeing computers go down one by one.
NHS staff shared screenshots of the WannaCry programme, which demanded a payment of $300 (£230) in virtual currency Bitcoin to unlock the files for each computer.
Throughout the day other, mainly European countries were reported to be infected.
A number of large Spanish firms including telecoms giant Telefonica, power firm Iberdrola and utility provider Gas Natural were hit, with reports that staff at the firms were told to turn off their computers.
Portugal Telecom, delivery company FedEx, a Swedish local authority and Megafon, the second largest mobile phone network in Russia, also said they had been affected.
People tweeted photos of affected computers including a local railway ticket machine in Germany and a university computer lab in Italy.
Reports say Russia had seen more infections than any other single country.
Russia’s interior ministry said it had “localised the virus following an attack on personal computers using Windows operating system”.
China has not officially commented on any attacks it may have suffered, but comments on social media said a university computer lab had been compromised.
Some security researchers have pointed out that the infections seem to be deployed via a program that spreads by itself between computers.
Most other malicious programmes rely on humans to spread by tricking them into clicking on an attachment harbouring the attack code.
By contrast, once WannaCry is inside an organisation it will hunt down vulnerable machines and infect them too.
This perhaps explains why its impact is so public because large numbers of machines at each victim organisation are being compromised.
Some experts say the attack may have been built to exploit a weakness in Microsoft systems that was identified by the NSA and given the name EternalBlue.
The NSA tools were stolen by a group of hackers known as The Shadow Brokers, who then attempted to sell the encrypted cache in an online auction.
However they subsequently made the tools freely available, releasing a password for the encryption on 8 April.
The hackers said they had published the password as a ‘protest’ about US President Donald Trump.
At the time, some cyber-security experts said some of the malware was real, but old.
A patch for the vulnerability was released by Microsoft in March, but many systems may not have had the update installed.
Microsoft said on Friday its engineers had added detection and protection against the malware. The company was providing assistance to customers.
A UK-based cyber-security researcher, tweeting as @MalwareTechBlog, said he had accidentally managed to temporarily halt the spread of the virus.
“So long as the domain isn’t removed, this particular strain will no longer cause harm, but patch your systems ASAP as they will try again,” he tweeted.