SAN FRANCISCO — Google Inc. is releasing its own Web browser in a long-anticipated move aimed at countering the dominance of Microsoft Corp.’s Internet Explorer and ensuring easy access to its market-leading search engine.

The Mountain View-based company took the unusual step of announcing its latest product on the Labor Day holiday after it prematurely sent out a comic book drawn up to herald the new browser’s arrival.

The free browser, called “Chrome,” is supposed to be available for downloading today in more than 100 countries for computers running on Microsoft’s Windows operating system. Google said it’s still working on versions compatible with Apple Inc.’s Mac computer and the Linux operating system.

Google’s browser is expected to hit the market a week after Microsoft’s unveiling of a test version of its latest browser update, Internet Explorer 8. The tweaks include more tools for Web surfers to cloak their online preferences, creating a shield that could make it more difficult for Google and other marketing networks to figure out which ads are most likely to appeal to which individuals.

Although Google is using a cartoonish approach to promote Chrome, the new browser underscores the gravity of Google’s rivalry with Microsoft, whose Internet Explorer is used by about 75 percent of Web surfers.

Google’s lead in the lucrative Internet search market is nearly as commanding, with its engine processing nearly two-thirds of the Web’s queries.

For the past few years, Google has been trying to take advantage of its search engine’s popularity to loosen Microsoft’s grip on how most people interact with personal computers.

The tensions between Microsoft and Google now seem likely to escalate with Google’s foray into Web browsing.