'Poison pill' defence to Twitter takeover

Poison pills are often used to defend against hostile takeovers.

Twitter's plan would take effect if Musk's roughly nine per cent stake grows to 15 per cent or more. 

Even then, Musk could still take over the company with a proxy fight by voting out the current directors. 

Twitter said the plan doesn't prevent the board from engaging with parties or accepting an acquisition proposal if it's in the company's "best interests."

Twitter had revealed in a securities filing Thursday that Musk offered to buy the company outright for more than $US43 billion ($A58 billion), saying the social media platform "needs to be transformed as a private company" in order to build trust with its users.

"I believe free speech is a societal imperative for a functioning democracy," Musk said in the filing. "I now realise the company will neither thrive nor serve this societal imperative in its current form."

Later on Thursday during an onstage interview at a TED conference, he went even broader: "Having a public platform that is maximally trusted and broadly inclusive is extremely important to the future of civilisation."