The neighborhoods of Bay Ridge and Dyker Heights in Brooklyn are hardly red-light districts.

Residents pride themselves on living in close-knit enclaves largely composed of families, far from the city’s honky-tonk grit; the local councilman calls the communities “bucolic.”

Yet, on Thursday, the police and prosecutors announced that a joint prostitution investigation in those very neighborhoods had resulted in the arrests of 19 people who operated or worked at 12 businesses that portrayed themselves as day spas or massage parlors but allegedly were fronts for paid sexual services. The seemingly overnight proliferation of such businesses in that area — a half dozen opened in the past two years along a six-block stretch of the shopkeepers’ spine of Fifth Avenue — was the subject of an article in The New York Times last month.

“There are backaches in Bay Ridge but not enough to support 19 massage parlors,” the police commissioner, Raymond W. Kelly, said at a news conference.

Investigators visited 19 parlors in the area but took action against 12, said Paul J. Browne, the chief Police Department spokesman; the others appeared to provide legitimate therapeutic services. The arrests took place on Wednesday, according to law enforcement officials, and of the 19 — 4 men and 15 women — 10 were charged with either prostitution or promoting prostitution. All 19 were charged with “unauthorized practice of a profession,” the authorities said. Massage therapists must be licensed by the New York State Education Department.