Evidence of the cyberattack that hit up to 100 countries continued to ripple around the world Saturday, with reports of Chinese students unable to access their graduation theses, British doctors canceling operations and passengers at train stations in Germany greeted by hacked messages on arrival and departure screens.

Individuals and organizations around the world were scrambling after the international attack, which began Friday and spread rapidly via email, to limit the damage or implement preventive measures.

The attack – thought to be one of the largest ever of its kind – was a form of ransomware that locks up computer systems and prevents access to data or systems until a payment is made.

It hit Britain’s beloved but creaky National Health Service particularly hard, causing widespread disruptions and interrupting medical procedures across hospitals in England and Scotland. During the attack on Britain’s NHS system, computer screens were locked by the malware that prompted the user to pay $300 in bitcoins or risk having their files erased.

On Saturday, it was still unclear who was behind the sophisticated attack.