Computers were infected with what is known as "ransomware" - software that freezes up a machine and flashes a message demanding payment to release the user's data.
A spokeswoman for Portugal Telecom said: "We were the target of an attack, like what is happening in all of Europe, a large scale-attack, but none of our services were affected".
Ransomware attacks are on the rise around the world.
"As the attack commenced on Friday night Taiwan time, many organisations, whether government or private sector, will only know the true impact on Monday morning when personnel return to work, turn on their computers, and possibly click on malware and/or otherwise discover that the organisation is the victim of ransomware", he said.
By Kaspersky Lab's count, the malware struck at least 74 countries.
"Once it gets in and starts moving across the infrastructure, there is no way to stop it", said Adam Meyers, a researcher with cyber security firm CrowdStrike.
The malware's name is WCry, but analysts were also using variants such as WannaCry.
Forcepoint said in a statement that the attack had "global scope", affecting organisations in Australia, Belgium, France, Germany, Italy and Mexico.
Telecommunications giant Telefonica was among the targets in Spain, though it said the attack was limited to some computers on an internal network and had not affected clients or services.
In Spain, major firms including Telefonica were hit, with employees told to shut down workstations immediately through megaphone announcements.
English hospitals and clinical commissioning groups (CCGs) in London, Blackpool, Hertfordshire and Derbyshire were among those to report problems.
"We are aware that a number of NHS organisations have reported that they have suffered from a ransomware attack".
British Prime Minister Theresa May said there was no evidence that patient data had been compromised in the attack, and that it had not specifically targeted the National Health Service.
NHS Digital is working closely with the National Cyber Security Centre, the Department of Health and NHS England to support affected organisations and ensure patient safety is protected.
"It had a countdown clock ticking down, stating that all data would be deleted unless a payment was received within that timeframe", he said.
Microsoft issued a patch on March 14 described as critical to users of Windows to fix that vulnerability, which CrowdStrike and Splunk said should protect users from getting infected by Wanna Cry.
"The ransomware can spread without anyone opening an email or clicking on a link".
LONDON/MADRID, May 12 (Reuters) - A global cyberattack leveraging hacking tools widely believed by researchers to have been developed by the U.S. National Security Agency hit worldwide shipper FedEx, disrupted Britain's health system and infected computers in almost 100 countries on Friday.
"More widely we ask people to use the NHS wisely while we deal with this major incident which is still ongoing".
In February 2016, a Los Angeles hospital, the Hollywood Presbyterian Medical Center, paid $17,000 in Bitcoin to hackers who took control of its computers for more than a week.
"HHS is aware of a significant cybersecurity issue in the United Kingdom and other worldwide locations affecting hospitals and healthcare information systems", Laura Wolf, chief of the critical infrastructure protection branch at the Office of the Assistant Secretary for Preparedness and Response (ASPR), said in a statement.
"Ambulances are being diverted to neighbouring hospitals".
A reporter from the Health Service Journal said the attack had affected X-ray imaging systems, pathology test results, phone systems and patient administration systems.
Caroline Brennan, 41, went to the hospital to see her brother, who had open heart surgery. "They said the system was down and that they can not transfer anyone till the computer system was back up so he is still in the theatre".