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An entire tactical unit of a US police department has quit after two officers accused of brutality were placed on unpaid leave, reports local media.

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.

According to the Buffalo News, the members have stepped down from the Emergency Response Team, but not the police department itself.

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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.

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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.

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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.

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