WASHINGTON, April 4 - The sexual exploitation of children on the Internet is a $20 billion industry that continues to expand in the United States and abroad, overwhelming attempts by the authorities to curb its growth, witnesses said at a Congressional hearing on Tuesday.

The witnesses, who testified at a hearing of the Oversight and Investigations Subcommittee, part of the House Committee on Energy and Commerce, said that sexual predators were preying on victims as young as 18 months by using instant messaging and Web cameras to meet, lure and digitally stalk children and to share pornography.

Internet technologies have the capacity to drive a wedge between children and their families, they said.

"Online predators befriend adolescents," said Dr. Sharon Cooper, a pediatrician at the University of North Carolina at Chapel Hill, who was one of the witnesses. "They become closer to them than some family members are."