LOS ANGELES — Amazon.com Inc. began selling applications for the Google Android smart phone in its new “Appstore” just days after Apple Inc. filed a lawsuit claiming that the digital storefront infringes on its “App Store” trademark.

Escalating the fight with Apple, Amazon said Tuesday that it was giving away an Android version of a popular game as part of the promotion for the new site. Normally the game, Angry Birds Rio, would sell for 99 cents, just as it does in Apple’s iTunes App Store.

The move was seen as a challenge to Google Inc.’s Android Marketplace for apps that run on smart phones and tablets using the Android operating system.

On its first day, Amazon’s Appstore had about 3,800 Android apps available for download.