Elon Musk is sharpening his knives at Twitter...'cause he's planning to wipe out half the workforce to put the company on the right path, or so he claims ... but employees have fired back with a preemptive lawsuit.

The new Chief Twit, as Musk anointed himself after his $44 billion purchase of the social media giant, fired off a chilling memo to his 7,500 employees Thursday.

The email -- which was only signed "Twitter" -- informed workers to stay away from all offices Friday as Musk begins to clean house, the New York Times reported. An exact head count was not provided in the email, but the NYT says half the staff is set to be fired.

The email leaves no doubt ... "In an effort to place Twitter on a healthy path, we will go through the difficult process of reducing our global workforce. We recognize that this will impact a number of individuals who have made valuable contributions to Twitter, but this action is unfortunately necessary to ensure the company’s success moving forward."

Twitter staffers read the tea leaves and filed a class action lawsuit late Thursday, asking a judge to declare any firings now would be in violation of California and federal law, in part because there was insufficient notice.

After receiving the ominous note, employees took to Slack -- the company's internal messaging service -- posting heart and salute emojis. Some workers were cut off from the internal computer system -- a sign their heads were on the chopping block.

Musk has already gone on a mini-firing spree after inking the deal to buy San Francisco-based company Oct. 27. As we reported, the world's richest man promptly gave walking papers to several top Twitter executives, including former CEO Parag Agrawal. Since then, Musk has reportedly chosen 50 engineers from his other businesses to pore over layoff lists of Twitter employees.

All sounds pretty ugly. Good luck.