Hospitals in Britain appeared to be the most severely affected by the attacks, which aimed to blackmail computer users by seizing their data. The attacks blocked doctors’ access to patient files and forced emergency rooms to divert people seeking urgent care.

Kaspersky Lab, a Russian cybersecurity firm, said it had recorded at least 45,000 attacks in as many as 74 countries.

It was not immediately clear who was behind the attacks, but the acts deeply alarmed cybersecurity experts and underscored the enormous vulnerabilities faced by disjointed networks of computer systems around the world.

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