Musk, the world's richest person, is suing OpenAI, its co-founder and chief executive Sam Altman and its president Greg Brockman, saying they betrayed him and the public by abandoning the ChatGPT maker's mission to be a benevolent steward of artificial intelligence for humanity, and transforming the non-profit into a profit-seeking juggernaut.
"If we make it okay to loot a charity, the entire foundation of charitable giving in America will be destroyed. That's my concern," Musk said in initial remarks, going on to describe his own life history.
Musk appeared calm, sometimes looking at and addressing the jury.
Bill Savitt, a lawyer for OpenAI and Altman, said it was Musk who saw dollar signs as he helped finance OpenAI's early growth and pushed it to become a for-profit business, one he might eventually lead as CEO.
Savitt said Musk wanted "the keys to the kingdom," and sued only after he failed and then in 2023 started his own AI business, xAI.
"What he cares about is Elon Musk being on top," Savitt said in his opening statement.
"We are here because Mr Musk didn't get his way at OpenAI."
OpenAI's lawyer also framed OpenAI's March 2019 creation of a for-profit entity as critical to letting it buy computing power and pay top scientists to stay competitive with Google's DeepMind AI lab.
Musk's lawyer Steven Molo told jurors in his opening statement it was the OpenAI defendants who wanted riches for themselves as OpenAI began drawing investors including Microsoft.
"The defendants in the case stole a charity, and we're asking you to hold them accountable," Molo said during his opening statement.
"It wasn't a vehicle for people to get rich."
Musk, the Tesla and SpaceX founder, is seeking $US150 billion ($A209 billion) in damages from OpenAI and Microsoft, one of its largest investors, with proceeds going to OpenAI's charitable arm.
He also wants OpenAI to revert to a non-profit, with Altman and Brockman removed as officers and Altman removed from its board.
Musk's claims include breach of charitable trust and unjust enrichment.
Before jurors were seated, US District Judge Yvonne Gonzalez Rogers admonished Musk after OpenAI lawyers complained about his posts on X on Monday in which he assailed Altman as "Scam Altman" and accused him of stealing a charity.
Rogers said she was loath to issue a gag order but urged Musk to "try to control your propensity to use social media to make things work outside the courtroom ... Perhaps you've never done that before".
Musk agreed to minimise his social media activity, as did Altman.
Both are expected to testify at trial, as is Microsoft chief Satya Nadella.
The trial could offer a window into some of the egos and personalities that shaped OpenAI as it evolved from a non-profit research lab in Brockman's apartment to a company worth more than $US850 billion.
It also risks complicating OpenAI's plans for a potential initial public offering by casting doubt on its leadership, and could intensify fears about AI technology more broadly.
OpenAI was co-founded by Musk and Altman in 2015 with a goal of developing AI to benefit humanity and fend off rivals such as Google.
Molo said "Elon became more worried" as the technology advanced, and collaborated with Altman to "develop AI safely" after a meeting with US President Barack Obama in 2015 did not address AI's risks.
Recruiting top AI scientists like Ilya Sutskever was part of that process, Molo said.
Savitt countered that AI safety was not a priority for Musk and that Musk denigrated OpenAI employees who focused on it.
"Jackasses is what he called them," Savitt said.
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