Sean Sullivan, security adviser to F-Secure, an worldwide cyber security company, said: "This is going to be a real problem for the NHS for months".
"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.
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.
"The key question" to consider is how an attack such as Friday's could originate "from a noncritical system such as email" and then spread to other systems, said Awais Rashid, a professor of software engineering at Lancaster University.
In Spain, the attacks did not disrupt the provision of services or networks operations of the victims, the government said in a statement.
"This may be because some expensive hardware (such as MRI scanners) can not be updated immediately, and in such instances organisations will take steps to mitigate any risk, such as by isolating the device from the main network".
Avast said the majority of the attacks targeted Russia, Ukraine and Taiwan.
The global efforts come less than a day after malicious software, transmitted via e-mail and stolen from the National Security Agency, exposed vulnerabilities in computer systems in nearly 100 countries in one of the largest "ransomware" attacks on record.
Six NHS trusts are yet to return to normal around 24 hours after an global cyber attack struck computer systems across the globe.
Two security firms - Kaspersky Lab and Avast - said they had identified the malicious software behind the attack 99 countries, although both said the attack hit Russian Federation hardest.
Pictures posted on social media showed screens of NHS computers with images demanding payment of $300 (AU$409) in Bitcoin, saying: "Oops, your files have been encrypted!"
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And all this may be just a taste of what's coming, a leading cyber security expert warned.
In the United States, package delivery group FedEx acknowledged it had been hit by malware and said it was "implementing remediation steps as quickly as possible".
After an emergency government meeting Saturday in London, Britain's home secretary said one in five of 248 National Health Service groups had been hit.
The malware was made available online on April 14 through a dump by a group called Shadow Brokers, which claimed past year to have stolen a cache of "cyber weapons" from the National Security Agency (NSA).
Finance chiefs from the Group of Seven rich countries were to commit on Saturday to joining forces to fight the growing threat of global cyber attacks, according to a draft statement of a meeting they are holding in Italy.
Krishna Chinthapalli, a doctor at Britain's National Hospital for Neurology & Neurosurgery who wrote a paper on cybersecurity for the British Medical Journal, said many British hospitals use Windows XP software, introduced in 2001. Microsoft has released software patches for the security holes, although not everyone has installed those updates.
In Germany, where rail operator Deutsche Bahn's passenger information screens were scrambled and some video surveillance cameras went dark, Interior Minister Thomas de Maiziere said that while the attack was particularly serious, it was also preventable.
USA software firm Symantec said the majority of organisations affected were in Europe, and the attack was believed to be indiscriminate. Ransomware is a type of malware that essentially takes over a computer and prevents users from accessing data on it until a ransom is paid, he said.
A spokesman for British Prime Minister Theresa May said she was being kept informed of the incident.
There are different variants of what happens: Other forms of ransomware execute programs that can lock your computer entirely, only showing a message to make payment to log in again.
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