New York is hurtling back to "the bad old days of high crime" under current Mayor Bill de Blasio, a major police union warned Tuesday, drawing a sharp rebuke from the Democrat.

"The degradation of our streets is on the rise," said Ed Mullins, president of the Sergeants Benevolent Association, urging the Democratic Party to choose another city to hold its 2016 convention.

In a full-page open letter in The New York Times, Mullins said that the city was "lurching backwards to the bad old days of high crime, danger-infested public spaces, and families that walk our streets worried for their safety."

Mullins, whose association claims to be the largest senior police officers' union in the United States, said that shootings had jumped 13 percent citywide since last year.

He accused de Blasio of making "dangerous choices" and said that the New York Police Department (NYPD) was "understaffed, overworked and underpaid."

De Blasio swiftly refuted the claims, declaring: "We are the safest big city in America. It's a well-established fact.

"We are the safest big city in America because we have the finest police force in America -- I've said it many times."

Crime was down 3.5 percent compared to this time last year, he said.

"We do have some problems with crime to address, for sure, we always have challenges," he said.

"But the good news is, and the clear evidence is, the NYPD is getting the job done and the city is safe -- in many ways safer than ever -- and we're continuing to make progress."

De Blasio accused the union of attempting to "advance their position in contract negotiations" and of fear-mongering.

"And I think that it's an irresponsible act on their part," he thundered.