An elderly man appears to be shoved by riot police during ongoing US protests. Reuters

Two Buffalo, New York, police officers who were shown on video shoving a 75-year-old protester to the ground are under investigation for potential criminal liability, a spokeswoman for the region's district attorney said on Friday.

"The Erie County District Attorney's Office continues to investigate the incident captured on video outside City Hall on Thursday evening that resulted in the injury of (a) protester," Kait Munro said in an emailed statement.

The incident on Thursday raised further questions about police behaviour, after the death of African-American man George Floyd in Minneapolis set off nationwide street protests.

The Buffalo video taken by a reporter from local radio station WBFO and posted online shows the white-haired man approaching a line of officers in riot gear. One officer pushes him with a baton and a second one with his hand. The sound of a crack is heard and blood trickles from the man's head.

New York Governor Andrew Cuomo said on Friday that he had spoken with the injured man, Martin Gugino, and was thankful to confirm he was alive. Cuomo said the police chief should fire the officers involved.

"You see that video and it disturbs your basic sense of decency and humanity," Cuomo told a daily briefing. "Why, why, why was that necessary? Where was the threat?"

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Gugino, who is white, could not be immediately reached for comment. PUSH Buffalo, which advocates for affordable housing, confirmed that Gugino is a member of the community group.

The video quickly went viral on social media, spurring outrage. Buffalo police initially said in a statement that a person "was injured when he tripped & fell,” WIVB-TV reported, but Capt. Jeff Rinaldo later told the TV station that an internal affairs investigation was opened. The police commissioner subsequently suspended two police officers without pay, Mayor Byron Brown said in a statement.

The mayor of the western New York city, who expressed he was "deeply disturbed” by the video, said the unidentified man was in "stable but serious" condition at a hospital.

"While it is early, thankfully he is expected to recover,” Erie County Executive Mark Poloncarz tweeted shortly after midnight.

Cuomo endorsed the officers' suspensions, tweeting that what was seen on video was "wholly unjustified and utterly disgraceful.” The office of State Attorney General Letitia James tweeted that they were aware of the video. US Sen. Charles Schumer called for an investigation, according to a statement reported by WIVB-TV.

"The casual cruelty demonstrated by Buffalo police officers tonight is gut-wrenching and unacceptable," John Curr, the Buffalo chapter director for the New York Civil Liberties Union, said in a statement, adding that it should be a "wake-up call” for city leaders to address police violence.