A previously convicted Brooklyn sex offender was arrested yesterday for apparently convincing an 18-year-old woman he was a cop before sexually assaulting her, police said.

The chilling tale dates back to around 6:30 a.m. on October 2nd, when the victim was walking on Gates Avenue near Broadway in Bed-Stuy to buy cigarettes. A brown 2010 Lexus LS460 pulled up beside her, and the driver, identified as 47-year-old Walter Barnes, asked where she was going and if she wanted a ride.

The victim got into the car, at which point Barnes allegedly produced a hand-held radio, which he spoke into using "police jargon." Investigators say he also took the opportunity to point to an NYPD van as it drove by, telling the victim he knew the officers inside.

Having successfully established his fabricated role as a cop, Barnes allegedly told the woman that if she did not perform a sexual act on him, she would be arrested. She complied, and police say Barnes drove to Aberdeen Street and Bushwick Avenue before making the demand again.

Finally, Barnes drove to Sherlock and Herkimer streets, where, according to investigators, he forced the victim into a sexual act for a third time. She did, however, manage to fight back when he tried to undress her, fleeing the vehicle for a nearby building.

Barnes was convicted of raping two girls—ages 14 and 17—in 2002, and is registered with the state as a "violent sexual offender."

He has been charged with rape, aggravated sex abuse and impersonating an officer.