Elon Musk gave a speech to enthusiastic Donald Trump fans Monday, hours after the presidential inauguration ... and he made a gesture that harkened back to a dark time.

At Capital One Arena, the Tesla and SpaceX CEO strode to the podium, laughing maniacally, and thanked the cheering crowd ... and then slapped his right hand hard to his chest, and extended it out in front of him, palm down -- in what looks very suspiciously like a Nazi salute. He then repeated the same action to those seated behind him.

The provocative signal has many people online and on-air comparing the expression to the WWII-era "Sieg Heil" salute to Nazi Germany dictator Adolf Hitler. That salute reportedly derives from an Ancient Roman Empire salute, more commonly performed with the slapped chest. Both salutes are generally associated with fascist history in Europe.

Musk called Trump's victory a "fork in the road of human civilization."

Musk was widely mocked last fall when he bounded up onstage to an introduction by Trump in October, jumping up and down and baring his midriff before a fawning speech to Trump, declaring himself "Dark MAGA."

But this time around, it looks a lot more sinister.