Microsoft Corp made its most vehement and public attack on Google Inc on Friday, calling its internet rival's actions potentially anti-competitive, and urging victims to file complaints to regulators.

The broadside comes days after a Microsoft-owned business, along with two other small online companies, complained to European Union regulators about Google's operations there. Microsoft is also fighting a plan by Google to digitize millions of books, currently under scrutiny by the Department of Justice.

Our concerns relate only to Google practices that tend to lock in business partners and content -- like Google Books -- and exclude competitors, thereby undermining competition more broadly, wrote Dave Heiner, Microsoft's deputy general counsel, in a blog published on the company's website on Friday.

Ultimately the competition law agencies will have to decide whether or not Google's practices should be seen as illegal, he wrote.

Google declined to comment on Microsoft's blog.

For the past two decades, Microsoft has been among the prime targets of competition regulators in the United States and Europe, over the way it handled its near monopoly of computer operating systems.

The world's largest software maker now seems keen to direct regulatory scrutiny onto Google, by far the world's biggest internet search company.

As Google's power has grown in recent years, we've increasingly heard complaints from a range of firms -- large and small -- about a wide variety of Google business practices, wrote Heiner.

Some of the complaints just reflect aggressive business stances taken by Google. Some reflect the secrecy with which Google operates in many areas. Some appear to raise serious antitrust issues.

Heiner said Google's way of working with advertisers and publishers makes it hard for Microsoft's competing Bing search engine to win search volume.

He suggested firms who feel they have been hurt by Google should complain to competition law agencies. The European Commission has not at this stage opened a formal inquiry into Google after it received complaints this week.

Microsoft's attack is certain to heat up relations between the two companies, which now compete on a broad spectrum of technology products, from software applications and mobile phone systems to internet search and e-mail programs.

(Reporting by Bill Rigby; Editing by David Gregorio)

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