New technologies, such as the specialized applications that run on Apple Inc's iPhones and fast-growing social networking services such as Facebook, are also threatening to undermine Google's position as the main gateway to online information, according to analysts.

Google shares are down roughly 26 percent from their 52-week high of $629.51.

Investors are concerned about increasing competition and rising spending at Google, as well as international expansion opportunities following Google's high-profile disagreement with China over censorship earlier this year.

Separately on Friday, Google's legal chief called for pressure on governments that censor the Internet, such as China and Turkey, arguing that blocking access to websites not only violates human rights but unfairly restrains U.S. trade.

Search Interrupted

Google executives said the new Google Instant technology would not change the order in which its search engine ranks Web pages or the way it displays advertisements alongside search results.

Still, the changes were a cause of concern among some of the websites and advertisers whose businesses can be significantly affected by changes to Google's search engine.

Kevin Lee, the chief executive of Didit, a search marketing firm that manages more than $160 million of advertising spending, said he was already fielding calls from a few clients.

The big question, Lee said, is whether Google's recommended search results will distract web surfers from the information they were initially looking for.

"Instead of refining searches, it may end up derailing searches. That's my big concern," said Lee.

"Advertisers have to watch if they're getting a different sort of visitor than they would have before," Lee said.

Google generated nearly $24 billion of revenue in 2009, predominantly from search advertising.

Google said the technology will also be available in many more countries, including France and the UK, in several weeks, and said it hoped to have a version that works on smartphones available in the coming months.