President Donald Trump suggested Tuesday that an elderly man seen bleeding from the head after he was pushed to the ground by US riot police was faking. "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 shoved hard and fell straight backwards after approaching an advancing line of police in helmets and body armour.

Video showed blood pooling under the back of the demonstrator'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 favourite 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 fuelled anger around the country at large protests against police brutality. Trump, who is behind in the polls five months ahead of his re-election battle in November, is campaigning on a slogan of "law and order."

He 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.