Tesla chief Elon Musk has launched a hostile takeover bid for Twitter, insisting it was a "best and final offer" and that he was the only person capable of unlocking the full potential of the platform.

The move throws another curve into a roller-coaster ride for Musk's volatile relationship with the global social media service, and raises many questions about what comes next.

"Twitter has extraordinary potential. I will unlock it," Musk wrote in a filing to US regulators made public Thursday, adding the site has the possibility to be "the platform for free speech around the globe." 

"The company will neither thrive nor serve this societal imperative in its current form. Twitter needs to be transformed as a private company," he added.

The world's richest person offered $54.20 a share, which values the social media firm at some $43 billion, in a filing dated Wednesday with the Securities and Exchange Commission.

Twitter's board said it would carefully review what it termed Musk's "unsolicited, non-binding" offer and decide on a course of action that was "in the best interest of the company and all Twitter stockholders."

Musk last week disclosed a purchase of 73.5 million shares - or 9.2% - of Twitter's common stock, an announcement that sent Twitter shares soaring more than 25%.

He was offered a seat on the board but turned it down over the weekend.

Musk went on to use Twitter as a stage to ask whether the social media network was "dying" and to call out users such as singer Justin Bieber, who are highly followed but rarely post.

"Most of these 'top' accounts tweet rarely and post very little content," the Tesla boss wrote, captioning a list of the 10 profiles with the most followers -- which includes himself at number eight, with over 81 million followers.

In other weekend tweets, Musk joked about dropping the "w" from Twitter's name and about converting its San Francisco headquarters to a homeless shelter "since no one shows up anyway."

He also suggested removing ads, Twitter's main source of revenue.

"He is such an entitled, privileged man I am not sure the Twitter he has in mind is a platform that will ultimately serve a majority of the people on it today," said Creative Strategies analyst Carolina Milanesi.

Musk has mused on Twitter about giving verified account checkmarks to everyone paying for premium subscription accounts, which cost $3 monthly.

"I invested in Twitter as I believe in its potential to be the platform for free speech around the globe, and I believe free speech is a societal imperative for a functioning democracy," Musk said in his filing.

Musk argued in the filing that Twitter needs to be transformed, and that he was offering a price that was 38% above its April 1 closing price, the last trading day before his growing stake was revealed.

'Not playing'

Musk breaks the mould as a business figure, even in the Silicon Valley world known for disrupting markets and changing lifestyles.

The serial entrepreneur's endeavours include driving a shift to electric vehicles with Tesla, private space exploration, and linking computers with brains.

His behaviour, however, has raised eyebrows, prompted laughs, and sometimes drawn condemnation or even litigation.

Jewish groups blasted his tweet comparing Canadian leader Justin Trudeau to Adolf Hitler over Covid-19 vaccine mandates and Musk later deleted the tweet without apologising.

Musk used Twitter to insult a British caver who was part of a dramatic effort to rescue boys trapped in a flooded cave in Thailand and to challenge Russian President Vladimir Putin to "single combat" over the invasion of Ukraine.

"It's get out the popcorn time as we expect many twists and turns in the weeks ahead as Twitter and Musk walk down this marriage path," Wedbush analysts said in a note to investors.

Twitter could be pressured to accept the offer by shareholders eager for the premium promised by Musk, or Twitter could seek a better offer elsewhere.

A host of questions likely to swirl around issues of financing, regulatory aspects and balancing Musk's time between his many companies.

Musk has also sparred repeatedly with federal securities regulators, who cracked down on his social media use after a purported effort to take Tesla private in 2018 fell apart.

"I am not playing the back-and-forth game," Musk wrote in his filing on the buyout offer. "I have moved straight to the end."