SAN FRANCISCO — In a direct challenge to Microsoft, Google announced late Tuesday that it is developing an operating system for PCs based on its Chrome Web browser.

The move sharpens the already intense competition between Google and Microsoft, whose Windows operating system controls the basic functions of the vast majority of personal computers.

In a post on its company blog, Google said the operating system would initially be aimed at netbooks, the compact, low-cost computers that have turned the PC world on its head. It said the open-source software, called Chrome OS, would be available in the second half of next year.

“Speed, simplicity and security are the key aspects of Google Chrome OS,” the blog post said. “We’re designing the OS to be fast and lightweight, to start up and get you onto the Web in a few seconds.”

Google has already developed an operating system for mobile phones, called Android. And several manufacturers of netbooks are also using that software.

Google has long promoted a vision of computing in which applications delivered over the Web play an increasingly central role, replacing software that runs on the desktop. In that world, applications run directly inside an Internet browser, rather than atop an operating system, the traditional software that controls most of the operations of a PC.

Last year, the company released the Chrome browser, which it described as a tool for users to interact with increasingly powerful Web programs, like Gmail and Google Docs, along with Web applications created by other companies. Since then, Google has been adding capabilities to Chrome, like allowing it to run applications even when a user is not connected to the Internet.

It is not clear how much work it would take for Google to turn Chrome into the central part of a full-fledged operating system. But in a recent interview, Marc Andreessen, who developed the first commercial browser and co-founded Netscape, said Chrome was already well along that path.

“Chrome is basically a modern operating system,” Mr. Andreessen said.

Google has also long customized a version of the Linux operating system for use internally.

The rise of netbooks has started to challenge some of Microsoft’s dominance in personal computing software. The first wave of netbooks relied on various versions of the open-source Linux operating system, and major PC makers like Hewlett-Packard and Dell have backed the Linux software.

In an unusual move, Intel, the world’s largest chip maker, has worked on developing a Linux-based operating system called Moblin as well.

The company has aimed the software at netbooks and smartphones in a bid to spur interest for its mobile device chip sold under the Atom brand.

To combat these efforts, Microsoft began offering its older Windows XP operating system for use on netbooks at a low price. In addition, the company has vowed that the next generation of Windows, Windows 7, due out this fall, will run well on the tiny laptops.

Netbooks have stood out as the brightest part of the PC market during the global economic downturn. Over all, PC sales have plummeted, while netbook sales have surged