Wainwright said Europol did not know the motive.
"We've seen the rise of ransomware becoming the principal threat, I think, but this is something we haven't seen before - the global reach is unprecedented", Europol Executive Director Rob Wainwright said on Sunday.
The attack has been found in 150 countries, affecting 200,000 computers, according to Europol, the European law enforcement agency.
He said that Russian Federation and India were hit particularly hard, in large part because the older Windows XP operating software is still widely used in the countries.
Experts say another attack could be imminent and have warned people to ensure their security is up to date.
The 5,500-strong Renault factory in Douai, northern France, one of the most important auto plants in the country, will not open on Monday due to the attack, sources told AFP.
Victims were asked for payment of $300 (275 euros) in the virtual currency Bitcoin.
There are fears of more "ransomware" attacks as people begin work on Monday, although few have been reported so far.
The US security firm Symantec said the attack appeared to be indiscriminate.
Experts advise people not to pay, as it would only encourage the attackers, there is no guarantee that they will unblock files, and may result in them gaining access to victims' bank details.
"It only guarantees that the malicious actors receive the victim's money, and in some cases, their banking information".More news: Cyber-Security Experts Fear the Progression Of 'WannaCry' Ransomware
The "Eternal Blue" tool developed by the National Security Agency had been dumped onto the public internet by a hacking group known as the "Shadow Brokers".
Major global companies said they also came under attack. "Organisations across Australia have been taking active steps to protect their networks over the weekend", the statement said.
He also warned hackers could upgrade the virus to remove the "kill switch" that helped to stop it.
U.S. Treasury Secretary Steven Mnuchin, at a meeting in Italy, said Saturday the attack was a reminder of the importance of cybersecurity.
Among those affected by the virus was Nissan, but the vehicle manufacturer said there had been no major impact.
The government is not legally bound to notify at-risk companies.
Radiology services at the Basingstoke and North Hampshire Hospital were affected in Friday's attack by computer hackers. FedEx said some of its Windows computers were breached.
Germany's rail operator Deutsche Bahn said its station display panels were affected.
Sixteen National Health Service organizations in the United Kingdom were hit, and some of those hospitals canceled outpatient appointments and told people to avoid emergency departments if possible. He tweeted: "The cyber threat is not over".
The danger will be discussed at the G7 leaders' summit next month.
Wallace said the government used to contract for computer services across the entire NHS but that in 2007 that was stopped and left to the individual trusts.