With airports bracing for Y2K problems and possible terrorism, the U.S. Customs Service has begun using new high-tech scanners that can see through passengers' clothing and search for contraband with an X-ray image that shows the naked body.

International travelers who are suspected of smuggling drugs or carrying weapons are being offered the body scanner as an alternative to a physical pat-down or frisk when they pass through ports of entry at airports across the country.

The scanner can display hidden guns, knives, batteries, digital watches, explosive materials and packages of drugs secreted under clothing. Supporters say scanners can help in the fight against terrorism and illegal drug importation.

But privacy advocates say the technology's capability to show the full external contours of the body, including male and female private areas, is an "electronic strip search" that erodes constitutional protections and is more invasive than a frisk, which is performed while a suspect is fully clothed.

Customs Commissioner Raymond W. Kelly says the body scanners give travelers the choice of avoiding the physical contact of an external body search at the hands of an inspector.

"The option is that we can pat you physically," he said, "or you can step in front of this machine. You don't have to do it." To ensure privacy, no image is recorded or preserved, he said. And the scanner operator is always the same sex as the person being scanned, Kelly said.

But Gregory T. Nojeim, legislative counsel for the American Civil Liberties Union, has been fighting the technology since it was first proposed as a security enhancement three years ago after TWA Flight 800 exploded off Long Island.

The body scanner can show "underneath clothing and with clarity, breasts or a penis, and the relative dimensions of each," he told an aviation safety conference shortly after the crash. "The system has a joystick-driven zoom option that allows the operator to enlarge portions of the image."

The image is not in photographic detail, but it provides a clear outline of the person's body.

The manufacturer of the BodySearch device says that the concerns are excessive. "You don't get a sharp line image," said Robert Peters, vice president of American Science & Engineering, of Billerica, Mass. Scanning private areas is necessary because "that's one of the places where people hide stuff," he added.

The Customs Service began installing body scanners over the last several months as part of Kelly's overhaul of inspection procedures in response to charges of racial profiling and a congressional hearing that followed. Black women in particular have complained that they were singled out for pat-downs, and a group in Chicago has filed a class action lawsuit against the agency.

The Customs Service was unable to provide numbers for those who have opted for scanning over frisks, and how many of these scans turned up contraband. Scanners were recently installed in New York, Miami, Atlanta, Los Angeles and Chicago at a cost of about $125,000 each.