www.Amazon.com Inc., the world's largest Internet retailer, began a digital-music download service to compete with Apple Inc.'s iTunes, selling restriction-free tracks from more than 20,000 record labels.

The MP3 service offers 2.3 million songs from more than 180,000 artists, www.Amazon.com said today. The songs, most priced from 89 to 99 cents, don't have software that limits how customers can store and play them.

The addition of a music-download service pits www.Amazon.com against iTunes, the world's most popular online-music store, and may offer the recording industry a chance to unlock Apple's dominance of the market. Adding the service also may lift www.Amazon.com's revenue as sales of compact discs decline.

"Amazon represents the best chance that the music industry has at building a competitor to iTunes," James McQuivey, a Forrester Research Inc. digital-media analyst, said. Apple accounted for 70 percent of all online music sales last year, according to market researcher NPD Group.

While CD shipments to U.S. retailers fell 13 percent last year, downloaded singles surged 60 percent, according to the Recording Industry Association of America. Sales of DVDs, CDs, books and other media accounted for 64 percent of www.Amazon.com's second-quarter revenue, down from 68 percent a year earlier.

"They have to embrace the download market and they want to be a leader," said Colin Sebastian, an analyst at Lazard Capital Markets LLC in New York. "They already have a strong customer base who buys music, so they should have a leg up." He rates www.Amazon.com shares "hold" and doesn't own any.

The music offerings were added to www.Amazon.com's Unbox video-download service.

www.Amazon.com shares gained in late afternoon Nasdaq trading. The stock has more than doubled this year.

www.Amazon.com said the service will be refined based on customer feedback from the test version announced today.

The tracks are sold without anti-copying programs known as digital-rights management software. Customers can play the songs on almost any device, including Apple's iPod and iPhone, Microsoft Corp.'s Zune, and Research in Motion Ltd.'s BlackBerry, and transfer them to CDs.

"Amazon's program is a clear head-to-head competition with Apple," said Scott Tilghman, an analyst with Soleil Securities Corp. in New York. "Amazon becomes the only major competitor to iTunes on iPods."

With reporting by Connie Guglielmo in San Francisco.