The billboards were bankrolled by state Sen. Eric Adams, who also made an online video to send his message: "You can raise your level of respect if you raise your pants."
Adams is the latest in a series of politicians and other public figures to lambaste the slack-slacks style that has been popular in some circles since the 1990s and amplified by rappers and other avatars of urban fashion.
The dropped-trousers trend has been debated in TV shows, city councils, school boards, state legislatures and courtrooms and even decried in song: Larry Platt became an Internet sensation earlier this year after he sang his original song "Pants on the Ground" during an "American Idol" audition.
Bill Cosby caused a stir by blasting baggy pants, alongside other things he considered missteps by black youths, at an NAACP event in 2004. President Barack Obama, as a candidate, came out against low-sitting trousers in 2008.
"Some people might not want to see your underwear. I'm one of them," Obama told MTV News.
Dallas officials embarked on a "Pull Your Pants Up" billboard campaign in 2007. Some schools have tightened dress codes to get students to tighten their belts. Last summer, a St. Petersburg, Fla., high school principal resorted to ordering thousands of plastic zip ties to help students hitch up their pants.
Some communities have tried outlawing saggy slacks, though such regulations have often faced questions about their legality.
Yet the trend has hung around. Adams decided he had enough after spotting a subway rider in particularly low-riding pair of pants a couple of months ago.
"Everyone on the train was looking at him and shaking their heads. And no one said anything to correct it," Adams said in a telephone interview this week.
So Adams, a black retired police captain first elected in 2006, tapped his campaign coffers for $2,000 to put up the billboards. He elaborated in his YouTube video, which juxtaposes images of minstrelsy and other racial caricatures with shots of sagging pants - all fuel for troubling stereotypes, in Adams' view.
The low-slung trousers trend is adapted from the unbelted and sometimes oversized look of prison uniforms, according to Mark-Evan Blackman, who heads the menswear department at New York's Fashion Institute of Technology.
Initially seen as invoking street credibility, the style has spread from inner cities to suburban malls - and into Blackman's classrooms, where he frequently finds himself telling students to hike up their trousers.
So does Tracey L. Collins, a former school principal who runs Fully Persuaded for Children and Families Inc. The New York-based organization aims to foster responsible decision-making and other social skills.
The swooning-slacks look "is one of those issues that impact young people greatly. They walk into classrooms, they walk into schools ... and people make an assessment about their appearance," said Collins, whose group is working with Adams on his "Stop the Sag" effort.
Communities from Lynwood, Ill., to Lafourche Parish, La., have passed laws imposing fines for too-low trousers.
Lawmakers in some places have considered such measures but rejected or dropped them amid legal questions. A plan to fine people for pants that exposed their underwear stalled in the Tennessee General Assembly last year, after the state's attorney general said it was "unconstitutionally vague." A Florida judge ruled a similar city law unconstitutional in 2008 after a 17-year-old in Riviera Beach spent a night in jail after being accused of having his underwear exposed.
Adams says he doesn't aim to legislate, just educate.
"I don't want to criminalize young people being young people," he said. "I'm trying to make sure we stand up and correct the behavior."
Still, some of the style's partisans aren't sure it merits a politician's attention.
"I think there's other things going on besides someone's pants being low," said James Scott, 27, of Brooklyn, his jeans sitting jauntily low on his hips.