The fatal shooting at a Brooklyn block party overnight — just steps away from where cops were doused with water last weekend — is proof the city is “slowly crumbling,” the Sergeants Benevolent Association said Sunday.

In a scathing email sent out to union members, SBA President Ed Mullins blamed Police Commissioner James O’Neill for the state of the city and called his removal.

“None of it is funny, 12 people were shot, someone is dead, and cops were hit with water. NYC is slowly crumbling. Don’t say you weren’t warned. Restore the NYPD – REMOVE O’NEILL!” Mullins wrote.

The email included one sergeant’s account of the aftermath of the shooting at the annual “Old Timers Day” party, which they described as “nothing short of a WAR ZONE” with hundreds of people refusing to listen to police.

The sergeant said gun violence was quite common at the event attended by thousands — despite Mayor Bill DeBlasio’s Tweet calling it a “peaceful neighborhood event.”

But this year was “one of the craziest incidents” with about 40 people arrested for disorderly conduct and obstruction, according to the sergeant.

The crowd of about 500, who were hanging out drinking and smoking weed after the shooting, were “extremely agitated” police were in a courtyard at New Lots and Mother Gaston Boulevard, according to the sergeant.

“People were yelling vulgar and vile things at all the Cops, such as WE WISH YOU WOULD DIE and a litany of other HORRIFIC TERMS,” the email read.

The account also alleges officers were ordered not to take action at the scene because Brooklyn District Attorney Eric Gonzalez was at the 73rd precinct — which the NYPD called “silly and false.”

“Assistant Chief Jeffrey Maddrey, Commanding Officer of Patrol Borough Brooklyn North was on scene, making all operational decisions and directing officers to maintain order and respond to the incident,” a spokesman for the NYPD said. “Arrests were made in the area when warranted, including a gun arrest after the shooting.”