Google Inc. said on Wednesday it was building Web software that runs online and off to let users work remotely on planes, trains, slow dial-up connections or even the most remote locations in the world.

The technology, called Google Gears, would allow users of computers, phones and other devices to manipulate Web services like e-mail, online calendars or news readers whether online, intermittently connected to the Web or completely offline.

By bridging the gulf between new Web services and the older world of desktop software, where any data changes are stored locally on users’ machines, Google is pushing the Web into whole new spheres of activity and posing a challenge to rival Microsoft Corp. leader in the desktop software era.

“The Web is great but it doesn’t work very well when you don’t have a Web connection,” Jeff Huber, Google’s vice president of engineering, said in an interview. “Gears addresses a functional gap on the Web.”

Google plans to make the Gears technology available for free as “open source” software, meaning other developers are free to use and enhance the software in their own products.