Google began selling its own mobile phone a much-anticipated move aimed at protecting its online advertising empire as people increasingly surf the Web on handsets instead of personal computers.

The Nexus One joins about 20 other mobile devices that already run on Android, the mobile operating system that Google introduced in 2007 to make it easier to connect to its services and other Web sites away from home or the office.

Google designed the touch-screen phone in partnership with Taiwan’s HTC Corp., which made the first Android-powered phone and will manufacture this one, too. Google will handle all sales online and has no plans to let consumers check out the Nexus One in retail stores.

The Nexus One has been in the hands of Google employees for the past three weeks, triggering media speculation and anticipation for the company’s first attempt to peddle a consumer electronics device.

The Nexus One’s $529 price tag is more than twice as much as the most powerful iPhone sold in conjunction with a two-year service plan from AT&T Inc.