WASHINGTON » Declaring child prostitution a "persistent threat" in America, the FBI said today that authorities had rescued 105 young people and arrested 150 alleged pimps in a three-day sweep in 76 cities.

The agency said it had been monitoring Backpage.com and other websites as a prominent online marketplace for sex for sale. Backpage.com said that it was "very, very pleased" by the raids and that if the website were shut down to the advertisements, the ads would be pushed to sites that wouldn’t cooperate with law enforcement.

The young people in the roundup, almost all of them girls, ranged in age from 13 to 17.

The largest numbers of children rescued in the weekend initiative, Operation Cross Country, were in San Francisco, Detroit, Milwaukee, Denver and New Orleans. The operation was conducted under the FBI’s decade-long Innocence Lost National Initiative. The latest rescues and arrests were the largest such enforcement action to date.

"Child prostitution remains a persistent threat to children across the country," Ron Hosko, assistant director of the bureau’s criminal investigative division, told a news conference. "We’re trying to put this spotlight on pimps and those who would exploit."

In Operation Cross Country, federal, state and local authorities cooperated in an intelligence effort aimed at identifying pimps and their young victims.

The FBI said the campaign has resulted in rescuing 2,700 children since 2003. The investigations and convictions of 1,350 individuals have led to life imprisonment for 10 pimps and the seizure of more than $3.1 million in assets.

In their efforts to identify child victims, investigators seek help wherever they can find it — in some cases from adult prostitutes, Hosko said. He said almost all the victims in sweeps like the one over the weekend are girls and that the profiles of the victims cut across racial lines and boundaries of wealth.

Social media are a common denominator in many of the rescues.

Last year, five members of the Underground Gangster Crips contacted teens at school or through Facebook, DateHookUp.com or other online social networking sites, enticing the girls to use their looks to earn money through prostitution.

As for websites, Liz McDougall, the general counsel for Backpage.com, said that if that site were shut down to the advertisements in question, the information that can lead to the rescues would be lost to law enforcement because the ads would be pushed to "offshore uncooperative websites."

"We feel very strongly that we’re doing the right thing, and we’re going to continue to do the right thing and we congratulate the FBI and everybody with the task forces involved in the program," said McDougall.

In earlier sweeps, child prostitution victims have been recovered at major sporting events — including the NCAA Final Four and Super Bowl, Hosko said.

In the 1990s, gangs took control of street prostitution across America; that forced pimps to move girls into sporting events where security existed, said Dr. Lois Lee, founder and president of Children of the Night, a nonprofit group that has rescued 10,000 children from prostitution since 1979.

Hosko said the plight of the young people often goes unreported to authorities because the children in many instances are alienated from their families and are no longer in touch.

Pimps operate wherever vulnerable potential victims can be found. Some are being recruited right out of foster care facilities, Hosko said.

For the past decade, the FBI has been attacking the problem in partnership with a private group, the National Center for Missing and Exploited Children.

John Ryan, the head of the center, called the problem "an escalating threat against America’s children."

The Justice Department has estimated that nearly 450,000 children run away from home each year and that one-third of teens living on the street will be lured toward prostitution within 48 hours of leaving home.

Congress has introduced legislation that would require state law enforcement, foster care and child welfare programs to identify children lured into sex trafficking as victims of abuse and neglect eligible for protections and services.

"In much of the country today, if a girl is found in the custody of a so-called pimp she is not considered to be a victim of abuse, and that’s just wrong and defies common sense," Sen. Ron Wyden, D-Ore., said during a Senate Finance Committee hearing last month. Wyden co-sponsored the legislation with Sen. Rob Portman, R-Ohio.