It is a seminal moment for the 16-year-old company, which emerged as one of the world's most influential public squares and now faces a string of challenges.

Musk, who calls himself a free speech absolutist, has criticized Twitter's moderation. He wants Twitter's algorithm for prioritizing tweets to be public and objects to giving too much power on the service to corporations that advertise.

Political activists expect that a Musk regime will mean less moderation and reinstatement of banned individuals including former President Donald Trump.  read more  Conservatives cheered the prospect of fewer controls while some human rights activists voiced fears of a rise in hate speech.  read more 

Musk has also advocated user-friendly tweaks to the service, such as an edit button and defeating "spam bots" that send overwhelming amounts of unwanted tweets.

Discussions over the deal, which last week appeared uncertain, accelerated over the weekend after Musk wooed Twitter shareholders with financing details of his offer.

Under pressure, Twitter started negotiating with Musk to buy the company at his proposed $54.20 per share price.  read more 

"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.

Former Twitter CEO Jack Dorsey weighed in on the deal late on Monday with a series of tweets that thanked both Musk and current Twitter CEO Parag Agrawal for "getting the company out of an impossible situation."

"Twitter as a company has always been my sole issue and my biggest regret. It has been owned by Wall Street and the ad model. Taking it back from Wall Street is the correct first step," he said.

Twitter shares rose 5.7% on Monday to finish at $51.70. The deal represents a near 40% premium to the closing price the day before Musk disclosed he had bought a more than 9% stake