Instagram-owner Meta has launched its Threads app, which experts say could attract Twitter users unhappy with recent changes to the platform.

The app is now available to download in over 100 countries including the UK, but not yet in the European Union because of regulatory concerns.

Users can create posts of up to 500 characters, and many features are similar to those found on Twitter.

But rivals have criticised the amount of data it may collect.

Meta calls the new app an "initial version", with extra features planned including the ability to interact with people on some other social media apps like Mastodon.

"Our vision with Threads is to take what Instagram does best and expand that to text," the firm says.

Despite Threads being a standalone app, users will log in using an Instagram account. Their Instagram username will carry over, but there is an option to customize their profile specifically for Threads.

Users will also be able to choose to follow the same accounts they do on Instagram, Meta says.

However, it is not yet clear whether disaffected Twitter users will want to move over to the Meta-run platform.

Meta whistleblower Frances Haugen said last year that the company had put "profits over safety" and criticised how the platform was moderated.

The company was also rocked by a scandal in which it allowed third parties, including the British political consultancy Cambridge Analytica, to access Facebook users' personal data.

In an apparent reference to this controversial past, Twitter owner Elon Musk joked on Monday "thank goodness they're so sanely run".

There are several other Twitter alternatives available, such as Bluesky and Mastodon.

However, Threads has a significant advantage because it is connected to Instagram, and the hundreds of millions of users already on that platform.

On Threads, we are told that posts can "easily" be shared between the two apps and can include links, photos, and videos of up to five minutes in length.

Users will have a feed of posts, which Meta calls "threads", from people they follow and recommended content.

They will be able to control who can "mention" them and filter out replies to posts that contain specific words.

Unfollowing, blocking, restricting or reporting other profiles will also be possible, and any accounts users block on Instagram will automatically be blocked on Threads.

But while Meta stresses the ties to Instagram, media coverage has focused on its similarity to Twitter, with some calling the app a "Twitter killer".

Meta says it takes "inspiration" from other products, although others put it less kindly. The Stories capability on Facebook resembles a Snapchat feature, and Reels on Instagram is unmistakably similar to TikTok.

Drew Benvie, chief executive of social media consultancy Battenhall, told BBC News that Threads looked a lot like Twitter: "The most obvious is that it's text-first, so users will be able to add photos, and comment on each other's posts.

"The length of the post looks like it's going to be roughly the length of Twitter as well. So a lot like Twitter".

Many commentators have suggested that it's no coincidence the app has launched amid upheavals at Twitter.

On Saturday, Twitter boss Elon Musk, restricted the number of tweets users could see, citing extreme "data scraping".

The move is the latest push by Mr Musk as he tries to get users to sign up to Twitter's subscription service, Twitter Blue.

Twitter also announced that its popular user dashboard, TweetDeck will go behind a paywall in 30 days' time.

Since Mr Musk took over, many users of Twitter have publicly expressed their dissatisfaction with the platform and with his stewardship, erratic behaviour and political views.

But alternatives have not so far caught on: Mastodon enjoyed only a limited surge in users. Bluesky, whose board members include Twitter co-founder Jack Dorsey, remains invite-only, limiting the numbers who can switch for now.

Last month, Mr Musk and Meta boss Mark Zuckerberg agreed – possibly in jest – to a cage fight. But now Twitter is facing heavyweight competition. Instagram has around two billion users, which dwarfs the 300 million that are believed to use Twitter – although its figures can no longer be verified.

"The thing about Twitter is that it's built a community and I think that's what's keeping a lot of the the hardcore Twitter users there. But it is small, so even if only one in 10 Instagram users tries out Threads, then that community is going to switch quite quickly", Mr Benvie told the BBC.

But while the Threads app will be available in the UK it is not yet available in the EU.

BBC News understands this is because of regulatory uncertainty, particularly around the block's Digital Markets Act, but the company will continue to look into launching in the EU.

That act, among other things, lays down rules on how large companies such as Meta can share data between platforms that they own. The sharing of data between Threads and Instagram is part of the issue.

Senior figures at rival platforms have also raised questions about the amount of data the platform may collect. Jack Dorsey Tweeted a screenshot of the types of data Threads might request including, location, browsing history, contacts, and financial information. "All your Threads are belong to us", he tweeted.

Elon Musk responded "yeah" – despite Twitter also taking similar information.

Meta has faced large fines over how it has previously handled user data on its platforms. In May it was hit with a record €1.2bn (£1bn) fine over data transfers to the US, and in September regulators slapped a €405m (£346m) fine on Instagram over its handling of children's data privacy.

Meta maintains protecting privacy is fundamental to its business. It says it uses data to do things like personalize feeds and help users stay safe on their apps and when people sign up to Threads they will agree to its privacy policy, just as they would for other products.