The Internet search-engine company Google is showing off its considerable heft, announcing this week that it has added one billion pages to its Web index. That move increases Google's breadth by about one-third as it braces for tougher competition from Yahoo and Microsoft. Google's search engine now spans 4.28 billion Web pages, up from 3.3 billion pages earlier this week.

The company, based in Mountain View, California, also has enlarged its index of Web images to 880 million, up from more than 400 million previously.

Microsoft is spending millions to develop a sophisticated search engine of its own to use on MSN.com in hopes of toppling Google as the king of search. For now, Microsoft's MSN site is relying upon Yahoo's search technology.

By rolling out its expanded search engine, Google stole some thunder from Yahoo, which announced Wednesday that it's ready to run its search engine without Google's help.

Yahoo, based in Sunnyvale, Calif., has been licensing results from Google's search engine since June 2000, helping to establish Google as the king of online search.

But Yahoo has long viewed Google, based in Mountain View, Calif., as a competitive threat and last year spent more than $2 billion on acquisitions to acquire more of the technology that it needed to run on its own search engine. Yahoo last month disclosed it would sever ties with Google before April, so Wednesday's announcement isn't a surprise.

Google's Web sites handled 35 percent of all Web searches in December, compared with 27 percent at Yahoo sites and 15 percent for Microsoft sites, according to the latest data compiled by comScore Media Metrix, a research firm. AOL and other Web sites owned by Time Warner have a 16 percent share. Those sites largely rely on Google for their search results.

Even with its expanded reach, Google still isn't close to capturing the constantly expanding constellation of online content. By some estimates, there are 10 billion pages on the Web.