A range of global institutions – including major banks, media and airlines – have reported suffering a mass IT outage.
Australia have been hit particularly hard with flights grounded, supermarkets experiencing checkout chaos and broadcast networks thrown off air when autocue, graphics and computers failed.
The US state of Alaska has also warned that its emergency services are affected.
The cause of the outage is unclear, but many of those affected have linked it to Microsoft's PC operating system.
An official Microsoft 365 service update posted to X earlier in the day said "we are investigating an issue that affects users' ability to access various Microsoft 365 apps and services."
A spokesperson for Australia's interior minister said the outage appeared to be related to a problem with global cyber security firm Crowdstrike, and the country's cyber security guard said there is no information to suggest it is an attack.
"Our current information is that this outage relates to a technical issue with a third-party software platform used by affected companies," they said in a statement.
Alaska officials said many 911 and non-emergency calls are not working properly.
The Australian telecommunications company Telstra has said the triple-0 call center - the main emergency contact in Australia - is not affected, but is working with other state emergency service providers to implement backup processes.
Meanwhile, at Sydney airport, information disappeared from departure boards. In a message to passengers in the domestic terminal, budget operator Jetstar said a "problem with Microsoft" meant it was unable to check in passengers or board its flights.
Virgin Australia also told its passengers there was a "complete ground stop" for flights.
Social media users have reported queues at Australian stores such as Woolworths, with payment systems down and problems accessing financial institutions such as National Australia Bank.
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