Mayor Bill de Blasio used one of the city’s gleaming cultural icons for “cheap propaganda” Wednesday — promising an NYPD photo exhibit at the Brooklyn Museum that turned out to be nothing more than a self-promotional stunt, critics griped.

In fact, the “exhibit,” contrasting New York’s bad old days with the safer, cleaner Big Apple of today, was quickly dismantled by City Hall staffers after a related discussion on record-low crime statistics and a Q&A with reporters.

No member of the public at large ever got to see it.

“I urge everyone to really look at it because it reminds us of all of the work that went into changing things,” de Blasio said at the press conference, hailing the display as “powerful” and “wonderful” — even as workers were working feverishly to remove the photos before the museum opened to the general public.

Critics derided the move as another example of de Blasio’s Communist-style propaganda.

Some of the images hung on the walls of the city’s third-largest museum, which holds roughly 1.5 million works, were simply bar graphs showing a downward trajectory of crime over the years — including a 4.1 percent drop in overall crimes in 2016.

A spokeswoman for the museum said of the display, “It was part of the press conference and taken down at the end.”

But earlier, the mayor had cited the exhibit as the reason for holding the press conference at the museum.

“I didn’t choose the location,” he told reporters. “The location was chosen because we had this wonderful exhibit that talked about the history, and we wanted to put this in historical perspective.”

The ruse raised eyebrows, particularly coming on the heels of another taxpayer-funded prop — a video of Broadway stars literally singing de Blasio’s praises in a year-end review of City Hall’s 2016 accomplishments.

“It’s just bizarre. They made a fake museum exhibit,” said a source. “It’s not even good propaganda — it’s cheap propaganda.”

Oddly, the crime stats alone would have been enough to showcase the NYPD’s achievement.

Crime was down in every borough. The total number of shooting incidents fell to 998, a sharp drop from the old record of 1,103 in 2013.

Deputy Chief Dermot Shea said there were 6,000 vehicle thefts, and “a good percentage of those are individuals who [left] their keys in the car.” In 1993, he noted, 110,000-plus vehicles were swiped.

NYPD spokesman J. Peter Donald claimed the exhibition was never sold as being part of the Brooklyn Museum.

“We never presented it that way . . . and the Brooklyn Museum didn’t present it that way,” he told The Post.

Instead, he said, the display was intended to generate conversations among NYPD brass, city staffers and more than 100 invited members of the public.

Officials said the exhibit was put up by NYPD and other municipal workers, and that the photos all belonged to either the agency or the city.

The museum didn’t charge for the use of space, and the NYPD couldn’t immediately provide the cost of staging the blink-of-an-eye exhibit.