Brazil's first lady Rosangela "Janja" da Silva on Saturday insulted Elon Musk, the billionaire owner of X, at an event ahead of the G20 summit in which she advocated tougher regulation of social media.
Speaking on a panel about disinformation, she appeared to be startled by a loud noise.
"I think it's Elon Musk," she said, adding, "I'm not afraid of you, fuck you, Elon Musk."
A video of the episode quickly spread on X, drawing a response from its outspoken owner.
"They are going to lose the next election," Musk wrote, adding a pair of laughing emojis.
The tycoon and soon-to-be member of US president-elect Donald Trump's administration has a complicated history with Brazil.
The country's supreme court suspended X for 40 days this year as part of a legal tussle over disinformation.
Leftist President Luiz Inacio Lula da Silva welcomed the suspension as legitimate measure against disinformation.
Musk is an ally of far-right former president Jair Bolsonaro, who has signalled his intent to run again in 2026 despite having been disqualified following his conviction for discrediting the country's electoral system.
Bolsonaro on Saturday posted a screen shot of the first lady making her controversial remark.
"We now have another diplomatic problem," he wrote.
The G20 summit of top economies takes place on Monday and Tuesday in Rio de Janeiro.
AFP