Google on Wednesday began selling digital music in a direct challenge to Apple's iTunes and Amazon's online shop. "Today we are happy to announce you can buy millions of songs right from Android Market," Google product manager Paul Joyce said at a press event in Los Angeles. The Internet search giant also announced that its Google Music service was coming out of test mode and is available free of charge to US residents.

Digital songs were available immediately for purchase online at the Android Market and the service will be rolled out to Android-powered smartphones and tablet computers in coming days, Google said.

"Doing a deal with Google for the sale of our repertoire is groundbreaking on a number of levels," said Robert Wells, president of global digital business at Universal Music. 

"We expect this to be a rich new revenue stream for our artists," Wells added while joining music industry peers on-stage at the event. Google signed three of the four major record labels -- Universal, EMI, and Sony Music -- with the holdout being Warner Music, home to Madonna and other artists. Coldplay, Shakira, and the Rolling Stones teamed with Google to celebrate the music store launch by making free or exclusive tracks available. One perk of the new service is the ability to share songs bought at the Android music shop with friends on the Google+ social network for one-time listening.

Google also designed the Music Store to allow independent musicians to be able to sell songs at prices of their choosing. Google in May launched a test version of Google Music, which allows users to store personal collections in the Internet "cloud" for streaming to smartphones, tablet computers or other gadgets.

Google avoided having to cut deals with music labels at the time by letting people store digital versions of songs they already own in online "lockers" which they can access using gadgets linked to the Internet.

As many as 20,000 songs can be stored at Google Music, which was pitched as "a completely legal" service akin to a person storing music collections on home computer hard drives. Google Music takes aim at a similar service launched in March by Internet retail powerhouse Amazon.com and is a long-coming step toward taking on Apple's iTunes digital content shop. With Amazon Cloud Drive and Cloud Player, users can upload digital music, photos, videos and documents to Amazon servers and access the files through Web browsers or phones and tablet computers running Google's Android software.