A massive cyber-attack using tools believed to have been developed by the US National Security Agency has struck organisations around the world.
Computers in thousands of locations have been locked by a programme that demands $300 (£230) in Bitcoin.
In April hackers known as The Shadow Brokers claimed to have stolen the tools and released them online.
Microsoft released a patch for the vulnerability in March, but many systems may not have been updated.
There have been reports of infections in 99 countries, including the UK, US, China, Russia, Spain and Italy.
Cyber-security firm Avast said it had seen 75,000 cases of the ransomware - known as WannaCry and variants of that name - around the world.
"This is huge," said Jakub Kroustek at Avast.
Many researchers say the incidents appear to be linked, but say it may not be a coordinated attack on specific targets.
Meanwhile wallets for the digital cryptocurrency Bitcoin that were seemingly associated with the ransomware were reported to have started filling up with cash.(Source: BBC)