In a video that went viral on Thursday, officers in the city of Buffalo, New York, were seen shoving an elderly man to the ground.
The 75-year-old was seriously injured, and taken to hospital.
All 57 officers in the riot squad have now reportedly resigned in protest at their two colleagues' suspension.
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According to the Buffalo News, the members have stepped down from the Emergency Response Team, but not the police department itself.
John Evans, president of the local police union, told the 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."
He added that Buffalo police are continuing to work with other law enforcement agencies.
The officers were enforcing a curfew as a result of protests that have spread nationwide since the death in Minneapolis last month of an unarmed black man, George Floyd, after a policeman knelt on his neck for nearly nine minutes.
What does the Buffalo video show?
The man is seen approaching a group of riot police as they advance.
Two of the officers appear to push the 75-year-old, causing him to fall backwards and hit his head.
An initial statement from Buffalo Police Department said the man 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.
Who is the elderly man?
His name is Martin Gugino, and authorities said he was taken to hospital, where he was in a stable but serious condition on Friday.
Mr Gugino is a longtime social justice activist in Buffalo, according to advocacy group Push (People United for Sustainable Housing).
He "has been a tireless fighter against injustice of all types for many years in our city both with Push and other grassroots organisations," they said in a statement, condemning police for his treatment.
This graphically illustrates the cruelty, callousness, and disregard for the lives of the public, that is common to most police in America. To them, human beings are garbage, trash, less than animals, since they treat their own canine helpers with great affection.
This objectification of living, breathing human beings is all of a piece with Josef Mengele, Martin Bormann, Heinrich Himmler, and thousands of other Nazis, especially the SS, who ran the concentration camps. If anyone does not believe America is a police state, here is the evidence for them on video. Not the first time - there are many other videos similar to this one and even worse, that I have seen and saved, demonstrating police indifference to the pain and suffering they inflict on others, whether white or black, young or old, all alike.
This indifference to the suffering of others is one of the hallmarks of the psychopath. This is who we have, supposedly protecting us from criminals - a criminal thuggery of brutal, callous, unfeeling, vicious, depraved psychopaths - a legalized gang of Gestapo thugs, a troop of bloodthirsty hyenas without a shred of compassion or moral responsibility.
It is what Hannah Arendt termed, when she observed Adolf Eichmann at his trial, "The Banality of Evil" - the casual, routine cruelty without even a shred of emotion - exactly as exemplified by someone kneeling on a man's neck while his life ebbed away, and continuing to kneel on his neck for two minutes and forty-six seconds even after he was already dead, as if to ensure he would not start breathing again, and all the while with an expression on his face of calm disinterest in the human life he was crushing out of existence.
Does anyone wonder at the outrage of the public? How would YOU like to have the life crushed out of you, with no more emotion than stepping on a cockroach? What is a human being? To these people, it is no more than a bug or a piece of trash.