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

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 social media's most iconic companies, despite its epic struggles.

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.

"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 spent the first few hours of the platform's launch replying to new users.

"One thing that's up is the number of world champion MMA fighters on Threads, especially now that you're here!" he wrote in a reply to American MMA fighter Jon Jones.

"Round one of this thing is getting off to a good start," he said in another.

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

In his first tweet in over a decade, Zuckerberg posted a Spiderman pointing at Spiderman meme in an apparent reference to the similarity of the two platforms.

Back 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, thereby sparing the new platform the challenge of starting from scratch.

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 communication channel 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.

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.

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

Musk hired advertising executive Linda Yaccarino to steady the ship, but she has not been spared his whimsy.

The Tesla tycoon said last week that he was limiting access to Twitter to ward off AI companies from "scraping" the site to train their technology.

Musk then 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.