The virus took control of users' files, demanding payments.
"The reason this is hitting so many computers at once is that they discovered a vulnerability in the most popular operating system in the world, in Microsoft windows", said John Carlin, former assistant attorney general for national security and an ABC News contributor. "We've never seen anything in this scale ..."
Russian Federation and Britain were among the worst hit countries.
President Donald Trump ordered his homeland security adviser, Tom Bossert, to hold an emergency meeting Friday night to assess the threat posed by the cyberattack, senior administration official told Reuters.
Cybersecurity experts believe the attack was carried out with the help of tools first developed by the U.S. National Security Agency for targeting terrorists and foreign adversaries, which was leaked to the public by a hacker group called The Shadow Brokers in April.
Security experts warned that further cyberattacks are likely.
In an interview with Britain's ITV, Europol Director Rob Wainwright said a cross-border investigation would be necessary to track down the culprits. "We're in the face of an escalating threat, the numbers are going up", he adds.
Wainwright said he was concerned that the numbers of those affected would continue to rise when people returned to work on Monday morning.
Europol spokesman Jan Op Gen Oorth said that the number of individual victims of the computer attack could be much higher.
Wainwright said the attack was indiscriminate, fast-spreading and unique, because the ransomware was being used in combination with a worm, which means the infection of one computer automatically could spread it through a whole network. The FBI and other law enforcement agencies worldwide say it's too early to determine who is behind this.
The agency said Monday that "very few" people have paid the ransom.
The effects were felt across the globe, with Russia's Interior Ministry and companies including Spain's Telefonica, FedEx Corp.in the US and French carmaker Renault all reporting disruptions.