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An outspoken witness of a South Slope sexual assault claims cops threw him in jail last week on a trumped up charge as payback for criticizing their investigation.

Donald Harrington spent a night behind bars for allegedly scratching a neighbor’s car three weeks after he shared details with this newspaper of a May 23 groping in which two cops from the 72nd Precinct released a possible suspect at the scene of the crime after neighbors held the man down and performed a citizens arrest.

After Harrington spoke out, two cops involved in the incident were placed on modified duty pending an investigation — but he says the story doesn’t end there.

On June 13, two officers from the same precinct knocked on the door of Harington’s 16th Street home.

“I heard them say my name — so I thought they were following up on the assault case,” he said.

But instead, he claims the officers slammed him against a car, threw his cellphone on the ground, and tossed him in jail overnight on charges of criminal mischief for purportedly leaving a nine-inch scratch on a neighbor’s car.

Harrington says cops drummed up the charges because he publicly lambasted the 72nd Precinct for letting a possible perp walk away after the victim of the sexual assault left the scene to seek refuge in her home just up the block.

“It was retaliation,” said Harrington, who claims at least one cop from the sexual assault investigation was involved in his arrest. “It’s completely unfounded.”

An NYPD spokesman says officers were simply responding to a complaint from a neighbor who filed a police report accusing Harrington of intentionally scratching his 2000 Honda Odyssey.

Police spokesman Brendan Ryan said it’s not uncommon for a suspect to spend the night in jail on felony criminal mischief charges and declined to comment on Harrington’s assertion that the officers who arrested him sought revenge.

“I can only comment on his arrest report,” he said.

Harrington, who pleaded guilty in 2008 for possession of a controlled substance, claims he was in class in Manhattan when the vandalism occurred and says video evidence will corroborate his alibi.

He plans to file a claim against the city.

A spokesman from the city’s Civilian Complaint Review Board, which handles complaints about police officers, couldn’t be reached by press time.

But the agency’s website describes police retaliation as a major breach of conduct — but not a particularly common offense.

“[It] violates department regulations, and could subject an officer to disciplinary or criminal charges. However, anecdotal evidence suggests that retaliation rarely occurs.”

But a neighbor who claims he saw officers use “excessive force” while arresting Harrington said this is clearly a case of retaliation — and it might make other witnesses of crimes hesitant to report what they have seen.

“It will contribute to people saying, ‘I just don’t want to get involved,’ ” said Bo Samajopoulos.

Reach reporter Natalie O'Neill at noneill@cnglocal.com or by calling her at (718) 260-4505.