In Kiev, the capital of Ukraine, A.T.M.s stopped working. About 80 miles away, workers were forced to manually monitor radiation at the old Chernobyl nuclear plant when their computers failed. And tech managers at companies around the world — from Maersk, the Danish shipping conglomerate, to Merck, the drug giant in the United States — were scrambling to respond. Even an Australian factory for the chocolate giant Cadbury was affected.

It was unclear who was behind this cyberattack, and the extent of its impact was still hard to gauge Tuesday. It started as an attack on Ukrainian government and business computer systems — an assault that appeared to have been intended to hit the day before a holiday marking the adoption in 1996 of Ukraine’s first Constitution after its break from the Soviet Union. The attack spread from there, causing collateral damage around the world.