Banks, airlines, telecommunications companies, television and radio broadcasters and supermarkets have been affected by IT systems problems.
Major US airlines have ground to a halt.
The outage appears to be affecting Windows PCs globally.
Users on the subreddit for cybersecurity firm Crowdstrike reported problems in India, the United States and New Zealand.
Users in Australia began reporting problems early this morning, saying they were locked out of their workstations, according to Sky News.
