Friday's wave of cyberattacks has hit 200,000 targets in at least 150 countries, the head of the European Union's police agency said, adding he feared that the number would grow when people return to work on Monday.
Europol Director Rob Wainwright told ITV's Peston on Sunday programme that the attack was indiscriminate.
Wainwright called the fast-spreading hack "unique" because the ransomware was being used in combination with a worm, meaning that the infection of one computer could automatically spread it through an entire network.
"The global reach is unprecedented. The latest count is more than 200,000 victims in at least 150 countries, and those victims, many of those will be businesses, including large corporations," he said.
He said few banks in Europe had been affected, having learned through the "painful experience of being the number-one target of cybercrime" the value of having the latest cybersecurity in place.
"We have been concerned for some time that the healthcare sectors in many countries are particularly vulnerable. They're processing a lot of sensitive data," he said.
Britain's state-run National Health Service was affected by the attack.
Wainwright said Europol was working with the FBI in the US to track down those responsible, saying that more than one person was likely behind it.
He said the cybercrime scene was increasingly going underground, meaning it was "very difficult" to identify the offender or their location.
"We're in a very difficult fight against these ever more sophisticated cybercrime syndicates that are using encryption to hide their activity," he said.
Wainwright said Europol provided free downloads of decryption programmes for most ransomware.
"Once we get to the bottom of this one, we'll make sure that this is available to people as well," he said.
It was believed to the biggest attack of its kind ever recorded.