WASHINGTON --- Hundreds of people have been arrested and 21 children rescued in what the FBI is calling a five-day roundup of networks of pimps who force children into prostitution.

The Justice Department says it targeted 16 cities as part of its "Operation Cross Country" that caps off five years of similar stings nationwide.

Many of the children forced into prostitution are either runaways or what authorities call "thrown-aways" -- kids whose families have shunned them. Officials say they are preyed on by organized networks of pimps who lure them in with shelter or drugs, then often abuse them until the children agree to work the streets.

"We together have no higher calling than to protect our children and to safeguard their innocence," FBI Director Robert Mueller said Wednesday. "Yet the sex trafficking of children remains one of the most violent and unforgivable crimes in this country."

Authorities arrested 345 people -- including 290 adult prostitutes -- during the operation that ended this week. Since 2003, 308 pimps and hookers have been convicted in state and federal courts of forcing youngsters into prostitution, and 433 victims have been rescued, Mr. Mueller said.

The cities targeted in this week's sting are: Atlanta; Boston; Dallas; Detroit; Houston; Las Vegas; Los Angeles; Miami; Montgomery County, Md.; Oakland, Calif.; Phoenix; Reno, Nev.; Sacramento, Calif.; Tampa, Fla.; Toledo, Ohio; and Washington.

The problem of 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 investigates child prostitution cases that cross state lines.

The cases aren't easy to convict.

In April 2006, for example, charges against a Nevada man resulted in a hung jury after his 14-year-old victim refused to testify against him. Months later, however, a second jury found Juan Rico Doss, of Reno, guilty of forcing two girls -- ages 14 and 16 -- to sell sex in Los Angeles, Sacramento, San Francisco and Oakland.

A University of Pennsylvania study estimates nearly 300,000 children in the United States are at risk of being sexually exploited for commercial uses, said Ernie Allen, the president of the National Center for Missing and Exploited Children.

"These kids are victims. This is 21st century slavery," he said.