EU commissioner Thierry Breton issued the warning after Musk sealed the $44 billion deal and fired Twitter CEO Parag Agrawal and two other executives.
The European Commission has reminded Elon Musk that he must follow EU rules following the confirmation that he has taken control of Twitter.
"In Europe, the bird will fly by our rules," Thierry Breton, the EU Commissioner for the Internal Market, said on the social media platform on Friday morning.
A few hours earlier, Musk tweeted to say "the bird has been freed," a reference to Twitter's logo.
On Thursday night, two people familiar with the deal said Elon Musk had taken control of Twitter and ousted the CEO, chief financial officer and the company’s top lawyer.
The sources wouldn’t say if all the paperwork for the deal, originally valued at $44 billion (€43 billion), had been signed, or if the deal has closed. But they said Musk is in charge of the social media platform and has fired CEO Parag Agrawal, CFO Ned Segal, and Chief Legal Counsel Vijaya Gadde. Neither person wanted to be identified because of the sensitive nature of the deal.
Bloomberg reported Musk will become CEO and will also reverse lifetime bans on Twitter users. Musk originally announced his plans to lift the lifetime Twitter bans in May, sparking fierce criticism as it would those banned on the social media platform include QAnon loyalists, COVID-19 deniers, neo-Nazis, and former US president Donald Trump.
The EU commissioner posted a link to a video published in May showing him with the Tesla CEO in his car factory in Austin, Texas. Musk said EU digital regulation was "exactly aligned" with his own thinking and he agreed "with everything" Breton had said on the measure which deals with platforms' obligations to moderate content.
The deal completion comes just hours before a deadline set by a Delaware judge to finalise the deal on Friday. The judge had threatened to schedule a trial if no agreement was reached.
The major personnel moves had been widely expected and almost certainly are the first of many major changes the mercurial Tesla CEO will make.
Musk privately clashed with Agrawal in April, immediately before deciding to make a bid for the company, according to text messages later revealed in court filings.
At about the same time, he used Twitter to criticise Gadde, the company's top lawyer. His tweets were followed by a wave of harassment of Gadde from other Twitter accounts.
For Gadde, an 11-year Twitter employee who also heads public policy and safety, the harassment included racist and misogynistic attacks, in addition to calls for Musk to fire her. On Thursday, after she was fired, the harassing tweets lit up once again.
Musk's changes will be aimed at increasing Twitter's subscriber base and revenue.
What else is Musk changing?
In his first big move earlier on Thursday, Musk tried to soothe leery Twitter advertisers, saying that he is buying the platform to help humanity and doesn’t want it to become a "free-for-all hellscape".
The message appeared to be aimed at addressing concerns among advertisers - Twitter’s chief source of revenue - that Musk’s plans to promote free speech by cutting back on moderating content will open the floodgates to more online toxicity and drive away users.
"The reason I acquired Twitter is because it is important to the future of civilisation to have a common digital town square, where a wide range of beliefs can be debated in a healthy manner, without resorting to violence," Musk wrote in an uncharacteristically long message for the Tesla CEO, who typically projects his thoughts in one-line tweets.
He continued: "There is currently great danger that social media will splinter into far right wing and far left-wing echo chambers that generate more hate and divide our society".
Musk has previously expressed distaste for advertising and Twitter’s dependence on it, suggesting more emphasis on other business models such as paid subscriptions that won’t allow big corporations to dictate policy on how social media operates. But on Thursday, he assured advertisers he wants Twitter to be “the most respected advertising platform in the world".
'Warm and welcoming to all'
The note is a shift from Musk's position that Twitter is unfairly infringing on free speech rights by blocking misinformation or graphic content, said Pinar Yildirim, associate professor of marketing at the University of Pennsylvania’s Wharton School.
But it’s also a realisation that having no content moderation is bad for business, putting Twitter at risk of losing advertisers and subscribers, she said.
“You do not want a place where consumers just simply are bombarded with things they do not want to hear about, and the platform takes no responsibility,” Yildirim added.
Friday’s deadline to close the deal was ordered by the Delaware Chancery Court in early October.
It is the latest step in a battle that began in April with Musk signing a deal to acquire Twitter, then tried to back out of it, leading Twitter to sue the Tesla CEO to force him to go through with the acquisition.
Musk strolled into the company’s San Francisco headquarters Wednesday carrying a porcelain sink, changed his Twitter profile to "Chief Twit," and tweeted "Entering Twitter HQ — let that sink in!"
Overnight the New York Stock Exchange notified investors that it will suspend trading in shares of Twitter before the opening bell on Friday, in anticipation of the company going private under Musk.
Musk is expected to speak to Twitter employees directly Friday if the deal is finalised, according to an internal memo cited in several media outlets.
Despite internal confusion and low morale tied to fears of layoffs or a dismantling of the company's culture and operations, Twitter leaders this week have at least outwardly welcomed Musk's arrival and messaging.
Top sales executive Sarah Personette, the company's chief customer officer, said she had a "great discussion" with Musk on Wednesday and appeared to endorse his Thursday message to advertisers.
"Our continued commitment to brand safety for advertisers remains unchanged," Personette tweeted Thursday. "Looking forward to the future!"
Musk's apparent enthusiasm about visiting Twitter headquarters this week stood in sharp contrast to one of his earlier suggestions: that the building should be turned into a homeless shelter because so few employees actually worked there.
The Washington Post reported last week that Musk told prospective investors that he plans to cut three-quarters of Twitter’s 7,500 workers when he becomes the owner. The newspaper cited documents and unnamed sources familiar with the deliberation.
Musk has spent months deriding Twitter's "spam bots" and making sometimes contradictory pronouncements about Twitter's problems and how to fix them. But he has shared a few concrete details about his plans for the social media platform.
Thursday's note to advertisers shows a newfound emphasis on advertising revenue, especially a need for Twitter to provide more "relevant ads" — which typically means targeted ads that rely on collecting and analysing users' personal information.