Pictures posted on social media showed screens of NHS computers with images demanding payment of 300 United States dollars worth of the online currency Bitcoin, saying: "Ooops, your files have been encrypted!"
The Government and NHS bosses are facing growing questions over why hospitals across the country were crippled by a global cyber attack amid suggestions preventative measures could have been taken "months ago".
"Unlike most other attacks, this malware is spreading primarily by direct infection from machine to machine on local networks, rather than purely by email", Lance Cottrell, chief scientist at the U.S. technology group Ntrepid.
Infections of the worm appeared to have fallen off significantly after a security researcher bought a domain that the malware was connecting to, by chance undermining the malware's effectiveness.
"The crisis isn't over, they can always change the code and try again", @MalwareTechBlog said.
Spanish telecommunications company Telefonica was among the many targets in Spain, Portugal Telecom and Telefonica Argentina were also targeted.
Also badly hit was Britain's National Health Service, which declared a "major incident" after the attack, which forced some hospitals to divert ambulances and scrap operations.
Rudd said the government did not know if the attack was directed by a foreign government.
Several cyber security firms said WannaCry exploits a vulnerability in Microsoft and that Microsoft patched this in March.
Experts say the virus, called Wanna Decryptor, exploits a vulnerability in Microsoft Windows software first identified by American spies at the National Security Agency (NSA).
"Today our engineers added detection and protection against new malicious software known as Ransom:Win32.WannaCrypt", Microsoft said in a statement on Friday, adding it was working with customers to provide additional assistance.
The spread of the ransomware capped a week of cyber turmoil in Europe that kicked off a week earlier when hackers posted a huge trove of campaign documents tied to French candidate Emmanuel Macron just 1-1/2 days before a run-off vote in which he was elected as the new president of France.
Authorities in Britain have been braced for cyber attacks in the run-up to the vote, as happened during last year's US election and on the eve of the French vote.
The attacks came in the form of ransomware, a technique used by hackers that locks users' files unless they pay the attackers a designated sum in the virtual currency Bitcoin.
Russia's interior and emergencies ministries, as well as the country's biggest bank, Sberbank, said they were targeted.
Separately on Saturday, finance chiefs from the Group of Seven rich countries will commit to join forces to fight the growing threat of global cyber attacks, according to a draft statement of a meeting they are holding in Bari, Italy.