Elon Musk reaches deal to buy Twitter for $44 bn, photo Justin Sullivan/Getty Images/AFP

Twitter famously served as a megaphone for former US president Donald Trump before the platform banned him, and Musk -- a self-proclaimed "free-speech absolutist" -- has said he wants to reform what he sees as the platform's over-zealous content moderation.

"Free speech is the bedrock of a functioning democracy, and Twitter is the digital town square where matters vital to the future of humanity are debated," Musk said in a statement released by Twitter.

"I also want to make Twitter better than ever by enhancing the product with new features, making the algorithms open source to increase trust, defeating the spam bots and authenticating all humans."

However advocacy groups were wary of the type of content Musk might allow on the platform, and Derrick Johnson, president of the NAACP civil rights organization, tweeted, "Do not allow Twitter to become a petri dish for hate speech, misinformation or disinformation. Protecting our democracy is of utmost importance."

At the White House, Press Secretary Jen Psaki said "no matter who owns or runs Twitter," President Joe Biden -- himself a Twitter user -- is "concerned about the power of large social media platforms."

Twitter stock closed 5.6 percent higher in New York trading.

- 'Back against the wall' -

Musk bought a nine-percent stake in Twitter earlier in April, then offered to buy the whole company outright, citing a mission of preserving free speech.

While the firm's board initially said it was reviewing his offer, it later rebuffed him and adopted a "poison pill" plan that would have made it harder for Musk to acquire a controlling position.

But last week, Musk -- whose immense wealth stems from the popularity of Tesla electric vehicles as well as other ventures -- said he had lined up $46.5 billion in financing comprised of debt, a margin loan and his personal fortune.

Dan Ives, an analyst at Wedbush Securities, predicted earlier in the day that since the board could not find another buyer, it would likely accept his offer.

"This basically put (their) back against the wall, they had to come to the negotiation table," he said in an interview on CNBC.

The publicly traded firm will now become a private company owned by Musk, who negotiated a purchase price of $54.20 per-share, Twitter said.

"Twitter has a purpose and relevance that impacts the entire world. Deeply proud of our teams and inspired by the work that has never been more important," the company's CEO Parag Agrawal said in a tweet.

- Trump downplays return -

Musk's efforts have raised hopes about the commercial potential of Twitter, which has struggled to achieve profitable growth despite its important spot in culture and politics.

Under Agrawal, who took over as Twitter CEO late last year, the company has made progress on new monetization features such as subscription products, Truist securities said in a note, adding that "short term, Musk's involvement at this stage runs the risk of disrupting those efforts."

Progressive groups were particularly concerned that Musk may permit a return to the platform by Trump, who was banned from Twitter after last year's assault on the US Capitol by his supporters seeking to overturn the 2020 presidential election result.

"The sale of Twitter to Elon Musk would be a victory for disinformation and the people who peddle it. Musk could unleash a wave of toxicity and harassment and undo Twitter's efforts to increase quality engagement and make its platform safer for users," Angelo Carusone, president of Media Matters for America, said in a statement.

In an interview with Fox News, Trump said "I am not going on Twitter," preferring instead to stay on his "Truth Social" media network.

"I hope Elon buys Twitter because he'll make improvements to it and he is a good man, but I am going to be staying on Truth," he said.