Chernobyl's radiation monitoring system is among the networks affected
Organisations and companies around the world are reporting a major cyber attack.
Experts believe it is a ransomware attack, similar to the WannaCry attack that occurred earlier this year.
Alan Woodward, a computer scientist at Surrey University told the BBC: "It appears to be a variant of a piece of ransomware that emerged last year.
"It was updated earlier in 2017 by the criminals when certain aspects were defeated. The ransomware was called Petya and the updated version Petrwap."
The virus demands $300 in bitcoins to let users access their data.
Chernobyl's radiation monitoring system is among the most serious digital networks affected.
The system has since been shut down, leaving employees to "go out and measure the [radiation] levels with hand-held meters".
Ukraine announced this morning that its government IT network had been crippled by an "unknown virus".
As it spread, it attacked services including several Ukranian banks and an electricity provider.
Kiev Airport was also hit.
British advertising agency WPP confirmed via Twitter that it has been impacted by the attack.
IT systems in several WPP companies have been affected by a suspected cyber attack. We are taking appropriate measures & will update asap.— WPP (@WPP) June 27, 2017
A Danish shipping company called Maersk also took to Twitter to confirm it had been targeted. It is thought that this impacts its offices in Ireland and the UK.
UPDATE 15:00 CEST pic.twitter.com/L5pBYvNQd3— Maersk (@Maersk) June 27, 2017
Russian, Indian and Dutch companies have also been hit, while Ukraine says it's suffered the biggest such hack in its history.
Ukraine's national bank, state power company and biggest airport have all been impacted by the attack, which targeted the government's infrastructure.
Deputy Prime Minister, Rozenko Pavlo, said members of the Ukrainian government were unable to access their computers.
“We also have a network 'down',” he wrote. “This image is being displayed by all computers of the government.”
Та-дам! Секретаріат КМУ по ходу теж "обвалили". Мережа лежить. pic.twitter.com/B74jMsT0qs— Rozenko Pavlo (@RozenkoPavlo) June 27, 2017
Additional reporting by IRN, Stephen McNeice