TOKYO — The worldwide “ransomware” cyberattack spread to thousands of more computers today as people logged in at work, disrupting business, schools, hospitals and daily life, though no new large-scale breakdowns were reported.

In Britain, whose health service was among the first high-profile targets of the attack on Friday, some hospitals and doctors’ offices were still struggling to recover.

The full extent of the damage from the cyberattack felt in 150 countries was unclear and could worsen if more malicious variations of the online extortion scheme appear.

The initial attack, known as “WannaCry,” paralyzed computers running factories, banks, government agencies and transport systems in scores of countries, including Russia, Ukraine, Brazil, Spain, India and Japan, among others. Among those hit were Russia’s Interior Ministry and companies including Spain’s Telefonica and FedEx in the U.S.

Britain’s National Health Service said about a fifth of NHS trusts — the regional bodies that run hospitals and clinics — were hit by the attack on Friday, leading to thousands of canceled appointments and operations. Seven of the 47 affected trusts were still having IT problems today.

Ciaran Martin, chief executive of the U.K.’s National Cyber Security Centre, has warned that more computers could be infected as doctors’ practices re-opened after the weekend.

Experts urged organizations and companies to immediately update older Microsoft operating systems, such as Windows XP, with a patch released by Microsoft Corp. to limit vulnerability to a more powerful version of the malware — or to future versions that can’t be stopped.

Paying ransom will not ensure any fix, said Eiichi Moriya, a cybersecurity expert and professor at Meiji University.

“You are dealing with a criminal,” he said. “It’s like after a robber enters your home. You can change the locks but what has happened cannot be undone. If someone kidnaps your child, you may pay your ransom but there is no guarantee your child will return.”

New variants of the rapidly replicating worm were discovered Sunday.