(changes dateline, adds background, quotes from RIAA CEO)

LOS ANGELES (Reuters) - Music Web site MP3.com Inc. said Tuesday it has counter-sued a powerful record industry association that last month accused the company of violating music copyrights.

MP3.com's complaint, filed Monday in San Diego Superior Court in California, accuses the Recording Industry Association of America (RIAA) and its chief executive officer, Hilary Rosen, of engaging in unfair business practices.

``Since inception, MP3.com has faced the increasingly aggressive tactics of the RIAA and its leadership,'' said Michael Robertson, chairman and CEO of MP3.com.

On Jan. 21, the music industry group, representing all five major record labels, filed suit in U.S. District Court in New York against MP3, accusing it of violating copyrights with new features that send music to any computer. That suit seeks to shut down the new services.

``We can no longer tolerate the bullying tactics of this not-for-profit trade association. After we get to the bottom of all of their actions toward MP3.com, we will vigorously pursue all of our legal remedies,'' Roberston said.

Rosen said in a statement Tuesday that the claims in MP3.com's lawsuit were ridiculous.

``This is a transparent attempt on the part of MP3.com to silence criticism of its infringing tactics. It won't work,'' she said.

``The lawsuit against MP3.com has nothing to do with MP3 technology. It has to with MP3.com, the company, taking music they don't own and haven't licensed and to offer new services to make money for themselves,'' she said.

MP3 shares closed unchanged at 28-1/2 on Tuesday -- far off its 52-week high of 105 and just above its low of 23-5/16.

MP3.com's core business is its vast library of more than 250,000 songs from 40,000 mostly unsigned musicians. Visitors to its site can download the songs for free and buy CDs and other merchandise from the artists.

The company is named after the popular digital music encoding format, MP3, which compresses songs down to a fraction of their former size so they can be sent quickly over the Internet while preserving near-CD quality sound. The company does not, however, own the rights to the technology.

The recording industry has balked at embracing the format because of its lack of copy protection, fearing it will make it easier for pirates to distribute instantly thousands of near- perfect digital copies of their most valuable hits.