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 "throwaways" — kids whose families have shunned them. Officials say they are preyed upon by organized networks of pimps who lure them in with shelter or drugs, then often beat, starve or otherwise abuse them until the children agree to work the streets.