Billionaire Elon Musk sparked controversy on Monday (Tuesday, Philippine time) after making a gesture at an event celebrating U.S. President Donald Trump's inauguration, with some observers calling it a "Nazi" or "fascist" salute.

Musk, the CEO of X, SpaceX, and Tesla, appeared on stage at the Capital One Arena in Washington, where Trump supporters had gathered for a rally. After thanking the crowd for returning the 78-year-old Republican to the White House, Musk tapped the left side of his chest with his right hand and extended his arm with his palm open, repeating the gesture for the crowd seated behind him.

Claire Aubin, a historian who specializes in Nazism within the United States, agreed with critics, calling the gesture a "sieg heil," or Nazi salute. "My professional opinion is that you're all right, you should believe your eyes," Aubin posted on X, aligning with those who viewed the gesture as an overt reference to Nazi ideology.

Ruth Ben-Ghiat, a historian of fascism, also described the gesture as a "Nazi salute — and a very belligerent one too," in a post on X.

The gesture quickly drew responses from Democratic Party members. Congressman Jimmy Gomez reacted by posting on X: "Well, that didn't take long."

Musk, who has since posted multiple times on his social media platform X, did not immediately address the controversy.

One rally attendee, however, defended Musk, suggesting the gesture was likely intended as a joke. "He's very humorous, and he uses a lot of sarcasm. So when he did that on the stage, I don't think he meant it," said Brandon Galambos, a 29-year-old pastor and tech worker.

Reports from Wired and Rolling Stone indicated that far-right personalities in the U.S. celebrated the gesture, with writer Evan Kilgore calling it "incredible."

In recent weeks, Musk has expressed support for Germany’s far-right AfD party and the British anti-immigration group Reform UK, drawing further scrutiny.

The Anti-Defamation League (ADL), an organization that combats anti-Semitism and has previously criticized Musk, defended his actions this time, calling the gesture an "awkward" move made in a moment of enthusiasm. "It seems that Elon Musk made an awkward gesture in a moment of enthusiasm, not a Nazi salute," the ADL said in a statement posted on X.

Democratic Representative Alexandria Ocasio-Cortez criticized the ADL's response, writing on X: "Just to be clear, you are defending a Heil Hitler salute that was performed and repeated for emphasis and clarity."

Historian Aaron Astor also disagreed with the Nazi interpretation. "I have criticized Elon Musk many times for letting neo-Nazis pollute this platform," Astor wrote on X. "But this gesture is not a Nazi salute. This is a socially awkward autistic man's wave to the crowd where he says 'my heart goes out to you.'"

In 2021, Musk revealed that he had been diagnosed with Asperger’s syndrome, a form of autism.