Google 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 news came along with word that the Google Music service was coming out of test mode and is available free of charge to US residents.

Digital songs were available immediately online at Android Market and the service will be rolled out to Android-powered smartphones and tablets in coming days, according to Google.

"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," he added while joining music industry peers on-stage at the event.