To put it in social media terms, the New York Police Department got trolled.

The nation’s largest police force learned the hard way that there are legions online devoted to short-circuiting even the best-intentioned public relations campaign — in this case, the NYPD’s Twitter invitation to people to post feel-good photos of themselves posing with New York’s Finest.

What #myNYPD got instead was a montage of hundreds of news images of baton-wielding cops arresting protesters, pulling suspects by the hair, unleashing pepper spray and taking down a bloodied 84-year-old man for jaywalking.

It was a fail of epic proportions, with the hashtag among the most-trafficked in the world Tuesday, creating a public relations nightmare for a new NYPD leadership intent on creating a more community-friendly force.

“We’ve seen instances before where a hashtag can become a bash-tag,” said Glen Gilmore, who teaches social media marketing at Rutgers University. “When you’re in the social space, it’s tough to predict what’s going to happen.”

A similar meltdown came last November when investment giant JP Morgan Chase, which had been paying billions in fines stemming from the financial crisis, asked followers on Twitter to post career advice questions. Among them: “Did you have a specific number of people’s lives you needed to ruin before you considered your business model a success?”

McDonald’s inadvertently ordered up some bad publicity in 2012 with its #McDStories campaign. Sample response: “I walked into a McDonald’s and could smell the Type II diabetes.”

The #myNYPD misfire comes at a time when new Police Commissioner William Bratton is trying to re-brand the department to counter criticism that it has been trampling on people’s civil rights. Last week, it disbanded an intelligence unit that spied on Muslim neighborhoods, and it has promised to reforms to the crime-fighting tactic known as stop and frisk.

Bratton acknowledged Wednesday that the Twitter campaign may not have been fully thought through.

“Was that particular reaction from the some of the police adversaries anticipated? To be quite frank, it was not,” Bratton said. “But at the same time it’s not going to cause us to change any of our efforts to be very active on social media. … It is what it is. It’s an open, transparent world.”

The #myNYPD traffic was holding steady on Wednesday, but the dialogue had shifted from mockery of the NYPD to an analysis of what went wrong, including the tweet “Social Media 101: Be careful what you ask for.”

Anthony Rotolo, a professor of digital communications at Syracuse University, suggested another appropriate response could have been #SMH — shake my head.

“A lot of time the eagerness to embrace social media tools overshadows our common sense,” Rololo said. “In other types of media, we would not so quickly jump to something like this without doing our groundwork first.”

Still, there was some evidence Wednesday that the outreach may bear fruit.

One person posted a photo of herself standing next to an officer on horseback in Times Square. Another posted a picture of two smiling officers on patrol and wrote, “These guys put their lives on the line every day. They deserve our respect and gratitude.”

(AP)