Two US policemen have been charged with second-degree assault after they were filmed pushing a 75-year old protester to the ground, seriously injuring him.

Aaron Torgalski, 39, and Robert McCabe, 32, pleaded not guilty in Buffalo, New York. They were released without bail, and face up to seven years in prison.

On Thursday, they were seen shoving Martin Gugino, who fell backwards on the pavement and started bleeding.

He remains in a local hospital in a serious but stable condition.

The two officers were enforcing a curfew in the city as a result of protests since the death of George Floyd in police custody in Minneapolis last month.

Aaron Torgalski and Robert McCabe, members of the Emergency Response Team, were suspended without pay after footage of the incident outside the City Hall went viral.

Fifty-seven of their colleagues - the entire unit - later resigned from the team in response to the officers' suspension.

On Saturday, a crowd of more than 100 supporters - including police officers and firefighters - protested outside the courthouse in Buffalo against the assault charges filed.

What are Buffalo prosecutors saying?

In a statement, Erie County District Attorney John Flynn said: "The two defendants, who are Buffalo Police officers, pushed a protestor outside of City Hall, causing him to fall and hit his head on the sidewalk."

He stressed that he was not taking sides in the high-profile case.

"We're on the same team here. We're all working each and every day to do justice, to keep our streets safe, to keep our communities safe.

"I'm partnered with law enforcement every day to do that. And when I have to prosecute one of my teammates it doesn't help the situation," Mr Flynn said.

What has been the reaction?

John Evans, president of the local police union, told the Buffalo News newspaper: "Our position is these officers were simply following orders from Deputy Police Commissioner Joseph Gramaglia to clear the square.

"It doesn't specify clear the square of men, 50 and under or 15 to 40. They were simply doing their job. I don't know how much contact was made. He did slip in my estimation. He fell backwards."

New York Governor Andrew Cuomo said on Friday the two officers should be fired, and called for the incident to be investigated for "possible criminal charges".

In a statement, Buffalo Mayor Byron Brown said: "We can confirm that contingency plans are in place to maintain police services and ensure public safety within our community."

An initial statement from Buffalo Police Department said the man - long-time social justice activist in Buffalo, according to advocacy group Push (People United for Sustainable Housing) - had "tripped" and fallen during a "skirmish involving protesters".

Police spokesman Jeff Rinaldo later attributed the statement to officers not directly involved in the incident.