Social media is awash with claims one of the Secret Service agents protecting President Donald Trump had prosthetic arms, with his real hands on the trigger of a gun under his coat.

The theory stems from analysis of video footage as President Trump, his wife Melania and son Barron went on an informal walkabout down Pennsylvania Ave after Saturday's inauguration.

The Trumps were surrounded by a phalanx of bodyguards in the standard-issue trenchcoats.

But one of the agents came in for particular scrutiny because his arms did not appear to move and his hands remained in the same position for several long minutes, the Daily Mail reports.

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The theory that he was holding a gun under his coat was only dispelled when video emerged of him apparently moving his arms and using his hands.

On Saturday there was high security as the presidential entourage of limousines travelled for more than 3km down towards the White House, occasionally getting out to greet the crowd.

The last President to be assassinated was John F Kennedy, in Dallas, in 1963 but Trump has polarised opinion in America on a scale never seen before and the Secret Service is clearly jittery about his security.

In March 1981, only two months after his inauguration, President Ronald Reagan escaped an assassination attempt in Washington.

White House press secretary James Brady was left paralysed by a bullet from would-be assassin John Hinckley, who was released from a psychiatric hospital last year.

In the footage from Friday the bald agent's eyes move constantly, scanning the crowds for signs of a potential assassin, but his arms do not move.

His right hand is splayed out and his left hand is bunched into a fist with one finger touching the other hand's pinky.

This led to internet speculation he was concealing a weapon under his jacket and had his real hand on the trigger.

There was more specific speculation that he had a Belgian-made FN-P90 sub-machine gun under his coat.

The agent's hands appear to be in the same position throughout the parade. Photo / AP
The agent's hands appear to be in the same position throughout the parade. Photo / AP

The FN-P90, which holds 50 rounds, is used by the Secret Service's Emergency Response Teams and is small enough to tuck underneath a winter coat.

The first to discuss the agent was an anonymous writer on the gaming blog Frag Hero who wrote on Sunday: "After yesterday's presidential inauguration, many members of the military and law enforcement community noticed something very unusual about one of Trump's bodyguards.

"The conclusion they reached was that he did indeed have tactical fake arms."

The blogger says the agent bears "an uncanny resemblance to Hitman Agent 47", a character played by Rupert Friend in a 2015 action movie based on a video game series.

But the Task and Purpose website said the theory was untrue and pointed to several images taken on Friday of the same agent with his hands in different positions.

Task and Purpose writer Adam Linehan, a former military man, points to one image of the agent getting out of a car: "The bodyguard, in the background of the image, exits one of the vehicles in the motorcade, and adjusts his tie and coat the way a man - a human man with functional arms - does whenever he's about to appear in public."