U.S. antitrust regulators have started a formal investigation into whether Google Inc abuses its market power by favoring its own services over those of rivals in online searches and through other practices.

Google, which dominates U.S. and global markets for search engine advertising, said on Friday on its official blog that it had been notified on Thursday by the U.S. Federal Trade Commission that it was reviewing Google's business practices.

The company has been accused of anticompetitive practices by other companies doing business online.

It's still unclear exactly what the FTC's concerns are, but we're clear about where we stand, Google blogged. Since the beginning, we have been guided by the idea that if we focus on the user all else will follow.

Complaints against Google have been filed with regulators on both sides of the Atlantic, many from companies specializing in vertical searches, such as price comparison websites.

U.S. antitrust regulators have expressed concern about Google's dominance of the Internet search industry, and the company has been under investigation by the European Commission since last November.

Shares of Google, the world's most popular search engine, began the year a touch above $600. The shares were trading at $474.38 on Friday afternoon.

Google has weathered antitrust challenges. It walked away from a search deal with Yahoo! Inc in 2008 when the U.S. Justice Department signaled that it was prepared to contest it.

Typically less than one out of every 10 investigations lead to enforcement. This investigation faces daunting odds, said David Balto, a former FTC policy director.

The complaints presented to the FTC are from disgruntled advertisers, not consumers. That is not a strong foundation to an antitrust case.

A New York judge has said that a deal between Google and publishers and authors to create a massive digital library was illegal, partially because it effectively gave Google the rights to books that are in copyright but whose authors cannot be located.

(Reporting by Diane Bartz; Editing by Derek Caney)