A statement from Instagram, which is also owned by Facebook, said people could log in to Threads using their Instagram accounts. Posts could be up to 500 characters and include links, photos and videos with a limit of five minutes.

The statement said the app was being rolled out in more than 100 countries for both iOS and Android devices, and that it would be made compatible with ActivityPub, the open social networking protocol established by the World Wide Web Consortium.

This would make Threads inter-operable with other apps built using the same protocol.

The announcement comes as Twitter is going through some convulsions, with owner Elon Musk's latest unpopular announcement being a limit on how many tweets the different classes of users could view each day.

"Everyone who is under 16 (or under 18 in certain countries) will be defaulted into a private profile when they join Threads," the announcement said.

"You can choose to follow the same accounts you do on Instagram, and find more people who care about the same things you do. The core accessibility features available on Instagram today, such as screen reader support and AI-generated image descriptions, are also enabled on Threads."