Friday's cyber-attack has affected more than 200,000 victims in 150 countries, Europol chief Rob Wainwright says.
He told the BBC the act was "unprecedented in its scale" and warned more people could find themselves affected on Monday morning.
The virus took control of users' files, demanding payments; Russia and the UK were among the worst-hit countries.
Experts say another attack could be imminent and have warned people to ensure their security is up to date.
Mr Wainwright said that the ransomware - software that blocks access to data until a ransom is paid - was combined with a worm application - a program that replicates itself in order to spread to other computers.
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Source - BBC