Elon Musk has launched an audacious bid to buy Twitter for $43.4bn, saying he wants to release its “extraordinary potential” to boost free speech and democracy across the world.

The Tesla chief executive and world’s richest person revealed in a regulatory filing on Thursday that he had launched a hostile takeover of Twitter. The news came just days after he bought a 9.2% stake in the social media company and was subsequently offered a seat on the board, but then refused to take up the position.

In a letter to Bret Taylor, Twitter’s chair, Musk said the site was not thriving as a company or a tool for improving freedom of speech, and “needs to be transformed as a private company”.

Musk, who has more than 80 million followers on Twitter, said if his offer was not accepted he would “reconsider my position as a shareholder” as he did not have “confidence in [Twitter’s current] management”.

“This is not a threat,” he added. “It’s simply not a good investment without the changes that need to be made.”

In the US Securities and Exchange Commission (SEC) filing on Thursday, Musk said he had offered to buy all Twitter’s shares for $54.20 each – a 54% premium to Twitter’s closing price on 28 January, the day before he started buying up his stake. However, after markets opened they only rose 1% in early trading.

“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 wrote in the letter to Taylor.