WASHINGTON (AFP) -- President Donald Trump suggested Tuesday that an elderly man bleeding from the head after he was shoved to the ground by US riot police was a far-left agitator faking the shocking incident.

"I watched, he fell harder than was pushed. Was aiming scanner. Could be a set up?" Trump asked on his massively followed Twitter account.

Two police officers in Buffalo, New York, were charged with assault after the incident last week in which protester Martin Gugino, 75, was pushed and fell straight backwards after approaching an advancing line of police in helmets and body armor.

Video showed blood pooling under the back of Gugino's head as he lay motionless. He is still in hospital.

It wasn't clear what Trump wanted to say by writing "aiming scanner" and that Gugino was "appearing to scan police communications in order to black out the equipment."

The president's tweet was apparently based on watching a report on OANN, a right-wing television network that specializes in conspiracy theories and has become a favorite at the White House.

In that clip, OANN promoted the idea of a "false flag provocation by the far left group Antifa." It claimed that "newly released video" showed Gugino "trying to scan police communication" with his phone.

The video only shows him holding his telephone in his right hand. Police scanner apps, which simply allow you to listen to radio traffic on police frequencies, are completely legal and common in the United States.

The video further fueled anger around the country at large protests against police brutality.

Trump, who is behind in the polls five months ahead of his reelection battle in November, is campaigning on a slogan of "law and order."

His Democratic opponent Joe Biden tweeted in response to the president by saying that his father "used to say there's no greater sin than the abuse of power. Whether it's an officer bloodying a peaceful protester or a President defending him with a conspiracy theory he saw on TV."

Trump has long been a fan of bizarre conspiracy theories, notably his extensive promotion of the fiction that Barack Obama was not born in the United States and therefore shouldn't have been president. (AFP)