Social media services Facebook, WhatsApp and Instagram have been hit by an outage lasting several hours.
All three services are owned by Facebook and could not be accessed over the web or smartphone apps.
Downdetector, which tracks outages, logged tens of thousands of outage reports for all three platforms from around the world.
Facebook’s chief technology officer Mike Schroepfer has tweeted his “sincere apologies” to those affected.
“We are experiencing networking issues and teams are working as fast as possible to debug and restore as fast as possible,” he wrote.
Some people also reported problems using Facebook’s virtual reality headset platform, Oculus, and apps which require Facebook logins were affected, including Pokémon Go.
An outage of this scale for such a long time is rare. Disruption in 2019 left Facebook and its other apps mostly inaccessible across the world for more than 14 hours.
As yet there has been no official reason given for the problem. But online network experts speculated it may involve an error with DNS, or the domain name system, for Facebook sites.
DNS is often compared to an address book or phone book for the internet, pointing web browsers to the computer system which serves the website they are looking for.
Previous issues with DNS led to widespread outages of several major sites earlier this year.
In one of those instances, it emerged that the blackout was caused by a single customer of a widely-used service who changed their settings, triggering a software bug affecting a huge number of websites.