US President Donald Trump has caused a headache for Republican politicians and sparked the ire of New York Governor Andrew Cuomo after sharing a conspiracy theory that a protester seriously injured by Buffalo police could be “an Antifa provocateur”.

Video of the incident, which happened late last week in Buffalo, in the state of New York, showed two officers pushing 75-year-old Martin Gugino to the ground.

He lay there, with blood coming from his head, as the officers marched past.

Mr Gugino was later taken to hospital, where he spent several days in intensive care. Two officers have been charged over the incident.

Last week Mr Trump blamed Antifa – an umbrella term for loosely defined far-left groups – for inciting violence during the Black Lives Matter demonstrations that have rocked the US.

The FBI has said there is no evidence the widespread protest movement has been hijacked by Antifa or any other “extremist” group.

“Buffalo protester shoved by Police could be an ANTIFA provocateur,” Mr Trump said on Twitter on Tuesday (US time).

“75 year old Martin Gugino was pushed away after appearing to scan police communications in order to black out the equipment. @OANN I watched, he fell harder than he was pushed. Was aiming scanner. Could be a set up?”

Tweet from @realDonaldTrump

One America News Network is a niche pro-Trump network that often peddles in unproven claims. It had apparently suggested the incident with Mr Gugino was a “false flag” incident staged by Antifa.

But his tweet drew immediate condemnation, with Mr Cuomo labelling a “reprehensible, dumb comment”.

“How reckless, how irresponsible, how mean, how crude,” Mr Cuomo said, adding that Mr Trump was pouring “gasoline on the fire” at a “moment of anguish and anger”.

“If he ever feels a moment of decency, he should apologise for that tweet, because it was wholly unacceptable,” he said.

Mr Trump’s tweet also caused consternation among some of his Republican colleagues.

John Randolph Thune, the Senate’s second most senior Republican, said Mr Trump had made a “serious accusation which should only be made with facts and evidence, and I haven’t seen any yet”, while Mitt Romney said he thought it was “shocking.”

“I won’t dignify it with any further comment,” he said.

Mr Gugino reportedly told USA TODAY Network – New York by text that he had “no comment other than Black lives matter. Just out of the ICU. Should recover eventually. Thx.”

Tweet from @keithgiles

But his lawyer told The Washington Post that Mr Gugino was a peaceful protester who loved his family, and “no one from law enforcement has suggested otherwise.

“We’re at a loss to understand why the president of the United States would make such a dark, dangerous and untrue accusation against him,” she said.

Jesuit priest James Martin said Mr Gugino was a peace activist and volunteer with the Catholic Worker movement.

“In case you’ve forgotten what’s in the Bible you were carrying, Jesus said, ‘Blessed are the peacemakers’, not the rumourmongers,” he tweeted.

It was a comment apparently aimed at Mr Trump, who posed with a Bible outside a Washington church a week ago – after protesters had been cleared from the area with tear gas and rubber bullets.

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The two Buffalo officers were charged with assault after shoving Mr Gugino.

Footage of the incident, captured by a local journalist, went viral and sparked outrage, with Mr Cuomo calling it “disgusting and horrendous”.

The incident came amid a massive protest movement across the US against police brutality and racism.

The movement was sparked by the death of African-American man George Floyd while being detained by white Minneapolis police officers in late May. Thousands attended Mr Floyd’s funeral in Houston on Tuesday (local time).

-with AAP