A trial pitting two founders of OpenAI - Sam Altman and Elon Musk - against each other has opened in California, with the sides presenting duelling narratives about the company's history and obligations to consumers.

Musk, wearing a dark suit and tie, was asked by one of his lawyers what the lawsuit was about when he took the stand.

"It's actually very simple," he said. "It's not okay to steal a charity... If it's okay to loot a charity, the entire foundation of charitable giving will be destroyed."

An OpenAI lawyer said the lawsuit was motivated by Musk seeking to kneecap a "competitor".

Musk and Altman were warned by the judge against using their platforms to attempt to influence the trial.

"We're here because Mr Musk didn't get his way at OpenAI," said OpenAI lawyer William Savitt. "Because he's a competitor, Mr Musk will do anything to attack OpenAI."

Musk lawyer Steven Molo reminded the nine jurors in Oakland to put aside their opinions of the two Silicon Valley billionaires and former friends.

"You all took an oath to put personal opinions aside," he said. "I know you will honour that oath."

Molo said that Musk had become more involved in AI as the technology advanced rapidly, growing concerned that "the government was not stepping up" to regulate it.

Musk's concerns increased, he added, after a 2015 meeting with then-President Barack Obama. His client, he said, had always believed that AI "wasn't a vehicle for people to get rich".

He went on to describe Musk's involvement in OpenAI, to which he had donated $38m (£28m) over several years, while it was operating at a non-profit.

"Without Elon Musk, there would be no OpenAI. Pure and simple," said Molo.

Musk objects to OpenAI's decision to open a commercial arm in 2018, years before it debuted the software ChatGPT, kicking off the commercial AI market.

In his opening arguments, Molo said that OpenAI CEO Altman and co-founder Greg Brockman "stole a charity".

Musk is asking for billions of dollars in what his lawyers call "wrongful gains" that he wants used to fund OpenAI's non-profit arm, and he wants to see a shake-up at the company including the ousting of Altman.

Musk's claims include breach of charitable trust and unjust enrichment.

OpenAI's lawyer said that Musk had used his investment to "bully" other OpenAI founders, and that he had wanted to merge the company with Tesla, which he also owns.

"The other founders refused to turn the keys of artificial intelligence over to one person," Savitt continued, adding: "When they refused to let OpenAI be absorbed" Musk "took his marbles and went home."

"Since he couldn't control OpenAI, he left it. He left it, he thought, for dead."

Speaking to Musk's legal case, he added: "Musk never cared about whether OpenAI was a non-profit."

OpenAI says Musk is motivated by jealousy and regret for walking away from the company. And as the race to artificial general intelligence (AGI) barrels full steam ahead, OpenAI accuses Musk of trying to derail one of his key rivals.

Musk's own platform, xAI, which makes the chatbot Grok, and has lagged behind competitors. xAI debuted in 2023, one year after ChatGPT hit the market.

OpenAI has argued that Musk understood the decision to open a commercial arm, and that he left the company only after failing to become its CEO.

Altman is also expected to testify during the trial.

Also on Tuesday, Judge Yvonne Gonzalez Rogers said she had decided against imposing a gag order to prevent participants from being able to talk about the trial outside of court.

On Monday, as the jury was chosen, Musk took to X, his social media platform, to refer to the defendant as "Scam Altman".

She asked Musk for a "clean slate" going forward, and that he "Try to control your propensity to use social media to make things worse outside this courtroom."

Defendants Altman and Brockman also agreed to do the same.

A verdict is expected to be announced in late May.