Almost hundred countries, including India, were hit by a massive cyber attack which experts said was carried out with the help of "cyber weapons" stolen from the US' National Security Agency.
"We've seen what happens when the US NSA (National Security Agency) ... develops hacking tools, effectively weapons for breaking in to ordinary people's computers then loses control of one of those exploits that has then been effectively weaponised by a criminal organisation that is now seeking to ransom people", he told reporters.
Europol said a special task force at its European Cybercrime Centre was "specially created to assist in such investigations and will play an important role in supporting the investigation".
It combined a known and highly risky security hole in Microsoft Windows, tardy users who didn't apply Microsoft's March software fix, and a software design that allowed the malware to spread quickly once inside university, business and government networks. It demands users pay Dollars 300 worth of cryptocurrency Bitcoin to retrieve their files, though it warns that the payment will be raised after a certain amount of time.
And all this may be just a taste of what's coming, a leading cyber security expert warned.
"It only guarantees that the malicious actors receive the victim's money, and in some cases, their banking information".
Cybersecurity experts have been working around the clock to try to halt the malware attack that is unprecedented in scale.
More than 75,000 similar attacks reportedly happened in nearly 100 countries, with Russia, Ukraine and Taiwan hardest hit, according to the cybersecurity firm Avast.
Lord Paddick, Liberal Democrat home affairs spokesman and a former Metropolitan Police deputy assistant commissioner, said Ms Rudd was "more suited to the era of analogue". Within hours, over 75,000 attacks have been detected worldwide, the company said.
Microsoft took the unusual step of reissuing security patches first made available in March for Windows XP and other older versions of its operating system.
"We need to get to the bottom of why the Government thought cyber attacks were not a risk, when a combination of warnings and plain common sense should have told ministers that there is a growing and unsafe threat to our cyber security", he said. Under current laws, they don't have to report the flaws to the company at risk.
Elliptic, a private security firm that investigates ransomware attacks, said that only about $32,000 had been sent to bitcoin addresses listed by the extortionists in ransom demands that flashed on screens of infected computers.
Kaspersky Lab confirmed that the company's protection subsystems detected at least 45,000 infection attempts in 74 countries, mostly in Russian Federation. Universities in Greece and Italy also were hit. It locks down all the files on an infected computer.
"You can't force businesses to patch critical Windows vulnerabilities", said Adrian Sanabria, founder of security firm Savage Security.
The attack appears to have been thwarted by private cybersecurity researchers who identified and triggered the malware's "kill switch", which halted the attacks before it spread throughout United States networks, a senior U.S. intelligence official confirmed, but it is unclear whether, the official said, a modified attack will soon be launched.
The researcher, tweeting as @MalwareTechBlog, said registering a domain name used by the malware stops it from spreading, though it can not help computers already affected.
Security researcher Troy Hunt also recommends making sure SMB ports (139,445) are blocked from all externally accessible hosts. "I have not slept a wink".
Britain's national health service fell victim, its hospitals forced to close wards and emergency rooms and turn away patients.
"It's important to understand that cyber attacks can be different from other forms of crime in that their sometimes highly technical and anonymous nature means it can take some time to understand how it worked, who was behind it and what the impact is", he told the BBC.