One block in South Harlem — just feet from Central Park — has become an open-air “flea market for drugs” straight out of the 1980s, a Post investigation has learned.

Brazen dealers are slinging dope in broad daylight on St. Nicholas and Lenox avenues between West 111th and West 112th streets — while addicts openly use crack and heroin and the NYPD lingers nearby, apparently doing nothing.

“During any time of the night or day, you’ll either see an active narcotic sale, active narcotic use, or someone out of their mind half-naked and in a zombie state,” said a 42-year-old professional who lives nearby with his wife and 8-week-old baby and who was too scared to give his name.

“I never would’ve thought one block from Central Park somehow open crack use is OK. It’s getting unbearable … my wife and I are thinking about moving to New Jersey at this point.”

On eight recent occasions between 8 a.m. and midnight, The Post observed at least a dozen drug dealers, both men and women, exchanging small plastic baggies for money during prolonged handshakes.

The Post also witnessed multiple people openly smoking small white balls of crack on the sidewalk moments after purchasing drugs, and in another case, saw a man passed out on a median in the middle of the street after smoking a drug from a glass pipe.

Nearby sat cop cars, but workers at the Wonder Fried Chicken & Pizza at 21 Lenox Ave. said the NYPD has not responded to their calls, even as the dealers have assaulted employees there and turned the eatery into their “headquarters.”

“I try to chase them out and they beat me up many times. They say, ‘If you don’t like it, you go. We are not going anywhere. This is our spot,’ ” said a worker, who’s called the NYPD “more [times] than the hair on my head.”

“The police has no power over them,” the worker said. “They do what they want to do … I complain to the officers. I asked why not take them, arrest them. They say, ‘When we go to court, the judge let them go.’ ”

The NYPD responded to the claims by saying: “The commanding officer of the 28 Precinct and the narcotics borough commander are actively working … to eradicate narcotics, arrest perpetrators, and target quality of life crimes in the area.”

A source said almost 200 arrests have been made in and around the location, mostly for drugs, and 47 summonses issued for street-level quality-of-life violations. Also, there is a 21 percent decrease in overall crime year-to-date in the 28th Precinct, the source said.

Despite this, the brazen drug activity in the area wasn’t hard to spot.

On one occasion, The Post witnessed a dealer throw bags of drugs onto the counter at Wonder Fried Chicken and exchange them for cash before walking back outside.

When a Post reporter recently interviewed staff at the chicken joint, they pulled her behind the counter so the dealers would think she was from the Health Department.

Two of the dealers dragged chairs outside to the sidewalk and watched her through the window while she was inside.

“They are going to ask [who you are],” the worker warned.

The Post first learned about the chaotic area — which has seen at least 10 felony assaults in 2019 — on Jan. 4, when an emotionally disturbed man jumped on top of a box truck, stripped off his clothes and danced naked, sending at least three ambulances and dozens of police officers to the scene.

That day, staff at Wonder told The Post the naked man provided just a glimpse into the daily madness they experienced from the constant drug dealing and use, and they begged for help.

A few doors down from Wonder is the Fine Fare supermarket, where The Post observed addicts exchange cans for cash at an outdoor bottle exchange while dealers stood by waiting, ready to trade drugs for the freshly earned cash.

On March 3 around 12:50 p.m., a pair of men were observed outside the Fine Fare, smoking crack out of a pipe, a resident told The Post.

The resident called 911 and when a pair of cops finally came, instead of searching the men or arresting them, they stayed inside their patrol cars and put on their emergency lights until the men walked away.

“I called a sergeant from the 28th Precinct and said, ‘What’s happening? I feel like I’m wasting my time. I’m not gonna call 911 anymore … why weren’t they arrested?’ ” the man questioned.

“He’s like, ‘No, no, no. That’s not the way things work. Unless the police officer sees them smoking crack, they can’t do anything. If they search them, they’re going to get sued, so they won’t do it,’ ” the sergeant said, according to the resident, in an apparent reference to post-stop-and-frisk policing.

Jennie Rivera, 37, who works at the Lenox Pharmacy at 27 Lenox Ave., said part of the problem is the unique set-up of the strip.

“This corner was almost designed to be this way,” Rivera said.

There’s the 110th Street 2, 3 train station, a liquor store, various restaurants and best of all, walking proximity to and from halfway houses and about a half-dozen homeless shelters, including a shelter for couples a block away that had a homicide in 2017.

“Police will hang out for a bit when it gets too rowdy, but that’s mainly on the weekends, and they never stay long,” Rivera said.

Additional reporting by Reuven Fenton and Joe Marino