More than 10 million people have signed up to Threads, Meta's rival to Twitter, within the first few hours of its launch, the company's CEO Mark Zuckerberg said Thursday.

The app went live on Apple and Android app stores in 100 countries at 2300 GMT on Wednesday, and will run with no ads for now, but its release in Europe has been delayed over data privacy concerns.

Threads is the biggest challenger yet to Elon Musk-owned Twitter, which has seen a series of potential competitors emerge but not yet replace one of the world's biggest social media platforms, despite its struggles.

"10 million sign ups in seven hours," Zuckerberg wrote on his official Threads account Thursday.

Accounts were already active for celebrities such as Jennifer Lopez, Shakira and Hugh Jackman, as well as media outlets including The Washington Post and The Economist.

Zuckerberg also offered a shot across the bow at Musk -- the pair are known to be bitter rivals, and have offered to wrestle it out in a cage fight.

On Threads, he wrote: "It'll take some time, but I think there should be a public conversations app with 1 billion+ people on it. Twitter has had the opportunity to do this but hasn't nailed it. Hopefully we will."

Twitter has said it has more than 200 million daily users.

 'Be kind'

Threads was introduced as a clear spin-off of Instagram, which offers a built-in audience of more than two billion users, sparing the new platform the challenge of starting from scratch.

Instagram chief Adam Mosseri told users that Threads was intended to build "an open and friendly platform for conversations."

"The best thing you can do if you want that too is be kind," he said.

Zuckerberg is widely understood to be taking advantage of Musk's chaotic ownership of Twitter to push out the new product, which Meta hopes will become the go-to platform for celebrities, companies and politicians.

"It's as simple as that: if an Instagram user with a large number of followers such as Kardashian or a Bieber or a Messi begins posting on Threads regularly, a new platform could quickly thrive," strategic financial analyst Brian Wieser said on Substack.

Analyst Jasmine Engberg from Insider Intelligence said Threads only needs one out of four Instagram monthly users "to make it as big as Twitter."

"Twitter users are desperate for an alternative, and Musk has given Zuckerberg an opening," she added.

Under Musk, Twitter has seen content moderation reduced to a minimum with glitches and rash decisions scaring away celebrities and major advertisers.

He has angered Twitter's most devoted aficionados by declaring that access to its TweetDeck product -- which allows users to view a fast flow of tweets at once -- would be for paying customers only.

EU 'many months' away

Meta has its legion of critics too, especially in Europe, which could slow the growth of Threads.

The company has been criticized for its handling of personal data, the essential ingredient for targeted ads that help it rake in billions of dollars in profits.

Mosseri said he regretted that the launch was delayed in the European Union, but had Meta waited for regulatory clarity from Brussels, Threads would have been "many, many, many, months away."

"I was worried that our window would close, because timing is important," he told the tech news site Platformer.

According to a source close to the matter, Meta was wary of a new law called the Digital Markets Act (DMA), which sets strict rules for the world's "gatekeeper" internet companies.

One rule restricts platforms from moving user data between products, as would potentially be the case between Threads and Instagram.

Meta was caught doing just that after it bought WhatsApp, and European regulators will be on high alert to ensure it does not do so illegally with Threads.

Globally, the Threads hashtag on Twitter has garnered over a million tweets, with many users jokingly suggesting people will return to Musk's platform.

"10 mins into threads app. Me coming back to Twitter," one user wrote, sharing a video of a man sprinting.

Others expressed privacy concerns.

"Meta loves to collect private information and I don't trust the way it treats private information," a Japanese user tweeted.

"I also have the impression that this is a company hated by EU, so I'm reluctant."

But some said they would permanently move to Threads.

One Threads user wrote: "Now I truly can say goodbye to Twitter forever."