For more than 10 years, Twitter has been recognizable for its blue and white bird logo, which became a symbol of the social network’s unique culture and lexicon. To “tweet” became a verb. A “tweet” referred to a post. “Tweeps” became the moniker for Twitter employees.

Late on Sunday, Elon Musk began getting rid of it all.

The tech billionaire, who bought Twitter last year, renamed the social platform X.com on its website and started replacing the bird logo with a stylized version of the 24th letter of the Latin alphabet. Inside Twitter’s headquarters in San Francisco, X logos were projected in the cafeteria, while conference rooms were renamed to words with X in them, including “eXposure,” “eXult” and “s3Xy,” according to photos seen by The New York Times.

Mr. Musk had long said he might make the name change, but he hastened the process in a tweet early Sunday morning when he declared that “soon we shall bid adieu to the twitter brand and, gradually, all the birds.” He has said he hopes to turn Twitter into an “everything app” called X that would encompass not only social networking, but also banking and shopping.

Early on Monday, Mr. Musk also shared a photo of a giant X projected on Twitter’s San Francisco office building with the caption: “Our headquarters tonight.”

The moves — which are continuing — are the most visible changes that Mr. Musk has made to Twitter since he closed the deal to buy the company in October. Behind the scenes, he has taken many steps to overhaul the firm, eliminating thousands of Twitter’s employees and changing the platform’s features, including badges that were meant to verify users, as well as the rules governing what can and can’t be said on the service.

Yet the name and logo change were impossible to ignore. And by starting to remove the Twitter name, Mr. Musk tossed out an entrenched brand that had been around since 2006 — when the company was founded — and that had delighted and frustrated celebrities, politicians, athletes and other users in equal measure. Twitter introduced its blue bird mascot in 2010 and updated it two years later.

Many Twitter users, who have spent years tweeting and building up their presence on the site, appeared alienated by the shift. “Has everybody seen the (eXecrable) new logo?” the actor Mark Hamill tweeted on Monday, with the hashtag #ByeByeBirdie. Others saw the move as Mr. Musk’s latest blow to the site, with some stubbornly saying they would still call the site Twitter and would continue to “tweet.”

When brands become verbs, it’s the “holy grail,” said Mike Proulx, a vice president and research director at Forrester, because it means they have become part of popular culture.

“The app itself has become a cultural phenomenon in all sorts of ways,” he said. “In one fell sweep, Elon Musk has essentially wiped out 15 years of brand value from Twitter and is now essentially starting from scratch.”

Mr. Musk risked the wrath of Twitter’s users even as he can ill afford to upset them. His company faces financial difficulties and increased competition, with rival Meta releasing an app this month for real-time, public conversations called Threads. The new app racked quickly up 100 million downloads in less than a week, though usage of the app is under scrutiny.

Mike Carr, a co-founder of the branding company NameStormers, said Mr. Musk’s X logo could be interpreted as having an ominous “Big Brother” tech overlord vibe. Unlike the blue bird, which he described as warm and cuddly but perhaps a bit dated and weighed down by bad press, the new logo was “very harsh,” he said.

Still, it conjured phrases like “X marks the spot” and could help Mr. Musk differentiate the platform from its Twitter baggage, Mr. Carr said.

“If they do this wrong and it was anybody other than Elon Musk, he’d be running a higher risk because people could start making fun of it,” said Mr. Carr, who has helped come up with names for thousands of clients, including CarMax, the used car company.

Mr. Musk has long been interested in the X name. In 1999, he helped found X.com, an online bank. The company changed its name after it merged with another start-up to form what would become the online payments company PayPal.

In 2017, Mr. Musk said he repurchased the X.com domain from PayPal. “No plans right now, but it has great sentimental value to me,” he tweeted at the time.

Tesla, Mr. Musk’s electric automaker, also has a sport-utility vehicle called the Model X. One of Mr. Musk’s sons, X Æ A-12 Musk, is often called “X” for short. The holding companies created to close the acquisition of Twitter were named X Holdings. Mr. Musk also leads an artificial intelligence company called xAI.

“I like the letter X,” he posted on Sunday.

Mr. Musk has shown a disdain toward Twitter’s previous corporate culture. He has quibbled with the number of bird references in the company’s internal team names and products. At one point, he changed the name of a crowdsourced fact-checking feature to “Community Notes” from “Birdwatch.” He recently also had someone cover the “w” on Twitter’s name at its San Francisco headquarters.

Among those who didn’t seem bothered by the change was Jack Dorsey, a Twitter founder and former chief executive. He said in a tweet on Monday that while a rebrand was not “essential” to achieving Mr. Musk’s vision, there was an argument for it.

“The Twitter brand carries a lot of baggage,” Mr. Dorsey wrote. “But all that matters is the utility it provides, not the name.”

Martin Grasser, a San Francisco-based artist who was part of a team in 2011 that helped design the most recent Twitter bird logo, said it was meant to convey “simplicity, brevity and clarity.” The goal was to have a logo that was as memorable as Apple’s or Nike’s, he said.

Mr. Grasser said Mr. Musk could do whatever he wanted with the brand, but “I hope the bird occupies a space in culture that is a happy memory or becomes one of those logos that belongs to culture rather than a company.”