Google Inc., owner of the most popular Internet search engine, will offer its own free Web browsing software to challenge Microsoft Corp.

The browser, known as Chrome, will be available as a test version for download today in more than 100 countries, Mountain View, Calif.-based Google said yesterday in a blog post. The software is designed to isolate flawed Web pages so users can close them without shutting down the entire browser.

Google is battling Microsoft for customers who use a browser for tasks such as e-mail, calendars, and word processing, applications that have traditionally been handled with software stored on a computer. A Web browser could help Google lure users to its programs and search engine, the president of Endpoint Technologies Associates in Wayland, Mass., Roger Kay, said.

“This gives Google another opportunity to protect its flank and to create a new branding position,” Mr. Kay said. “The browser is a broader platform than they currently have.”

The market for Web-based software may reach $160 billion by 2011, including revenue from advertising, Merrill Lynch & Co. said in a May report.

“We realized that the Web had evolved from mainly simple text pages to rich, interactive applications and that we needed to completely rethink the browser,” said the Google blog post, credited to the vice president of product management, Sundar Pichai, and the engineering director, Linus Upson. Microsoft’s Internet Explorer has about 72% of the Web browsing market, followed by Mozilla Corp.’s Firefox with 20% and Apple Inc.’s Safari with 6.4%, according to research firm Net Applications of Aliso Viejo, Calif.

Microsoft released a new version of Internet Explorer last week for testing. The software lets users control whether it saves the sites they’ve visited. Mozilla said last week it extended an agreement through 2011 to keep Google as its default search engine.