Twitter has threatened to take legal action against Threads, a new rival app from Meta that has gained tens of millions of users since its release on Wednesday.

On the same day, an attorney representing Twitter, Alex Spiro, sent a letter to Meta CEO Mark Zuckerberg accusing Threads of engaging in "systematic, willful, and unlawful misappropriation of Twitter's trade secrets and other intellectual property."

The letter, which was first reported by Semafor , accuses Meta of hiring dozens of former Twitter employees with the intention of creating a "copycat" platform.

"Twitter intends to strictly enforce its intellectual property rights, and demands that Meta take immediate steps to stop using any Twitter trade secrets or other highly confidential information," Spiro wrote. "Twitter reserves all rights, including, but not limited to, the right to seek both civil remedies and injunctive relief without further notice."

Meta Communications Director Andy Stone dismissed the accusations in a Threads post on Thursday.

"No one on the Threads engineering team is a former Twitter employee — that's just not a thing," he wrote.

Over on Twitter, owner Elon Musk replied to a post reporting the letter by writing, "Competition is fine, cheating is not."

Twitter has seen a host of challenges from similar microblogging platforms since Musk first acquired the platform for $44 billion last year. But none have grown as quickly as Threads, where, Zuckerberg reports, more than 70 million users had signed up by Friday morning.

The app's user interface looks and operates much like Twitter, with buttons to like, reply, repost or quote a thread. But users have bemoaned the lack of some classic Twitter features, like hashtags and direct messaging.

Meta responded to NPR's request for more information by pointing to Stone's response on Threads.

Since March, Twitter's communications team has sent a standard automatic response to emails from the press, containing nothing but a poop emoji.