"The governments of the world should treat this attack as a wake up call", Microsoft's president and chief legal officer, Brad Smith, wrote in a blog post about what is being called the largest ransomware attack ever.
He told the BBC the act was "unprecedented in its scale" and warned more people could find themselves affected on Monday morning.
It exploited a vulnerability in the Windows operating system believed to have been developed by the National Security Agency, which became public last month.
Experts say the spread of the virus had been stymied by a security researcher in the United Kingdom hackers have issued new versions of the virus that cyber security organizations are actively trying to counter and stamp out.
The world's biggest ransomware attack levelled off overnight after wreaking havoc in 150 countries, as Russian President Vladimir Putin called it payback for the U.S. intelligence services.
A fifth of regional hospital associations in Britain's National Health Service were affected and several still had to cancel appointments on Monday, as doctors warned of delays as they can not access medical records.More news: Jack Hunt reacts on Twitter to Sheffield Wednesday draw with Huddersfield Town
Here's a look at how malware and ransomware work and what people can do if they fall victim to attacks.
"In particular, making sure that our data is properly backed up and making sure that we are using the software patches, the anti-virus patches, that are sent out regularly by manufacturers".
More recent Microsoft systems, including Windows 10, 8 and 7, can also be infected by the ransomware virus.
Computer systems at companies and hospitals in dozens of countries were hit Friday, apparently part of a huge extortion plot.
The company said it had released a security update back in March to protect Windows system computers against such attacks, but said many computers remained unpatched globally. The virus took control of users' files and demanded $300 (£230) payments to restore access.
Microsoft (MSFT.O) released patches in April and on Friday to fix a vulnerability that allowed the worm to spread across networks.More news: Lawmakers urge Trump to avoid picking a partisan for FBI job
He said it was unclear so far how it had started but ransomware generally spread via a few different methods.
Though a British security researcher "MalwareTech" managed to stop the spread of the virus, hackers have issued new versions that cybersecurity organisations are trying to counter.
"Work is ongoing to restore our IT systems, but in the meantime we are unable to proceed with numerous appointments scheduled for the day".
India is on high alert, monitoring critical networks across sectors like banking, telecom, power and aviation to ensure that systems are protected against the attack that has claimed victims in more than 150 countries over the weekend. The official would not identify which systems but said no federal government entities were hit.
In China, "hundreds of thousands" of computers at almost 30,000 institutions and organisations were infected by late Saturday, according to Qihoo 360, one of China's largest providers of antivirus software.
However Home Secretary Amber Rudd said it was important to remember that it was not just the NHS which had been affected.More news: Senator on Trump recordings: 'You can not be cute about tapes'
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