SAN FRANCISCO — Hoping to crack down on music piracy, five major record labels have quietly begun selling CDs containing technology that foils attempts by customers to copy the songs onto blank discs or computer hard drives.

The new strategy is not widespread yet, and most of the CDs out so far are being sold in Europe. The labels will not say which artists' works have been digitally padlocked.

The so-called stealth CDs play fine in stereos. But if someone tries to turn the music into MP3 files or copy it onto a blank CD, the copied version will not work or the result will sound so bad it's not worth sharing.

Most movie DVDs and videocassettes already use anti-copying technology.

Music fans have long made personal copies of purchased music — an activity legally sanctioned as "fair use." But the music industry contends that giving consumers the ability to make perfect digital copies that can be shared online goes far beyond fair use.

Daniela Mohor, a 26-year-old University of California-Berkeley graduate student who often makes copies of CDs for friends, thinks the new strategy is ridiculous.

"With tapes it was the same thing; you could record them and the industry didn't fall apart," she said. "I understand the industry is trying to protect its profits, but they should find a solution that will benefit both sides."

Warner, EMI, Universal Music Group, BMG and Sony are the labels believed to be involved. Only BMG and Sony said copy-protected CDs have actually been put on shelves — in Europe. Other details were not released.

The new CDs aim to provide an impervious barrier against the Internet music free-for-all that Napster and CD burners made so popular.

Phil Leigh, a digital media analyst for Raymond James and Associates, thinks such efforts will fail.

"Music needs to be portable," he said. "It needs to get into the car and into your portable device, and the industry will find it's a losing battle unless they provide copying."

The Israeli company Midbar Tech Ltd. said more than 1 million protected CDs have been released in Europe, including 10,000 Sony CDs released last year.

One of Midbar's three copy-protection options permits tracks from a CD to be copied to a computer for listening — but not moved to another PC or shared online. Another Israeli company, TTR Technologies Inc., has developed a way to add distortion to unauthorized digital copies.

Macrovision Corp. spokeswoman Miao Chuang said as many as 200,000 CDs protected by the company's anti-piracy technology have been sold in the United States.

Sami Valkonen, senior vice president of new media and business development at BMG Music, said his company is not looking to eliminate music from computer desktops but wants to control how it gets there.

"A lot of people enjoy their music digitally these days," Valkonen said. "We would never ever in the U.S. have the situation where people would not have the ability to enjoy their music as digital files."

The methods used to protect CD content include disguising the disc's directory of songs so that popular copying programs cannot find the tracks to extract.

SunnComm Inc. of Phoenix used this technology on 50,000 copies of a Charley Pride CD released in May. The company said the CDs withstood the tests of would-be copiers, but fans complained the CDs were unplayable on their home stereos.

Kevin Gray, a recording engineer for Hollywood, Calif.-based Future Disc Systems who does work for all five major record labels, said adding distortions to music is a mistake that would be "very audible" to clued-in listeners.

Gray said he, too, thinks people will find a way around most CD copy protections. SunnComm chief executive Peter Jacobs agreed.

"We liken ourselves to a padlock on the music," he said. "There will always be people with bolt cutters out there willing to steal things."