WASHINGTON — More than 600 adults have been arrested and 47 children rescued in a three-day roundup targeting people who force children into prostitution.

The FBI said the roundup by federal, state and local law enforcement occurred in 29 cities, adding that the raids dismantled 12 large-scale prostitution operations run through call services, truck stops, casinos and Web sites.

At a news conference announcing the results of Operation Cross Country II, FBI Deputy Director John Pistole said the arrests were made possible by intelligence gathered during a similar series of raids in June.

"Sex trafficking of children remains one of our most violent and unconscionable crimes in this country," Pistole said.

The 47 rescued children ranged in age from 13 to 17, and all but one are female. Of these, Pistole said, 10 had been reported to the National Center for Missing and Exploited Children.

A total of 642 people were arrested. The FBI says they include 73 pimps and 518 adult prostitutes.

The operation was part of a larger, five-year initiative that has led to the recovery of 575 children and the dismantling of 36 criminal operations since June 2003.

Child prostitution has taken on a new urgency in recent years with the growth of online networks where pimps advertise the youngsters to clients. The FBI generally gets involved in child prostitution cases that cross state lines.

A University of Pennsylvania study estimated that nearly 300,000 children in the United States are at risk of being sexually exploited for commercial purposes.

"Make no mistake, the minors who were interdicted as a part of this operation are victims of the most extreme form of sex trafficking," said acting Assistant Attorney General Matthew Friedrich.

"It sadly remains the case even now in 2008 that there remain instances of children in the United States manipulated into prostitution by pimps willing to make a fast buck based on the most vulnerable among us," he said.