A plan by the New York Police Department to use Twitter to boost its image seems to have backfired.
Users were asked to tweet a photo of themselves with officers and add the hashtag #myNYPD as part of a social media campaign.
But instead of a steady stream of friendly photos, the hashtag was quickly adopted by users posting images of possible police aggression.
The NYPD said: "Twitter provides an open forum for uncensored exchange."
The original tweet was posted on the NYPD's Twitter feed on Tuesday. Featuring two smiling officers and a member of the public, it encouraged users to send in similar photos.
But while several people did so, the hashtag was also picked up by others who used it to identify tweets containing photos of the NYPD in more hostile situations.
By Wednesday, the hashtag had become one of Twitter's top trending terms.
One photo showed a man being pushed down on to a car bonnet. It was from March 2013 and followed protests in Brooklyn over the death of 16-year-old Kimani Gray who was shot by police.
The protest group Occupy Wall Street tweeted an image of an NYPD police officer advancing towards a crowd with a baton raised.
Many of the photos appeared to be taken by professional photographers at incidents in New York City rather than users' own images.
One from the Associated Press showing a man being held down on the floor by two officers appeared in several tweets.
The NYPD issued a statement on Tuesday evening in response to the activity: "The NYPD is creating new ways to communicate effectively with the community. Twitter provides an open forum for an uncensored exchange and this is an open dialogue good for our city."
Other Twitter interactions that have backfired include US Airways posting an explicit photo in response to a customer's tweet and McDonald's using a hashtag to highlight its farmers that quickly got taken over by people sharing their bad experiences of the burger chain.