The banner Friday reads “De Blasio, our backs have turned to you.”
That’s a reference to police officers who turned their backs to the mayor at a hospital last week after two officers were fatally shot.
Former police officer-turned-activist John Cardillo says that “hundreds” of retired and current officers paid to have the banner flown.
Officers Rafael Ramos and Wenjian Liu were killed by a gunman who claimed he was upset at the deaths of black men at the hands of white officers.
Police unions have blamed de Blasio for allowing protests they say foster an anti-police sentiment.
A de Blasio spokesman called the banner divisive.