Microsoft Corp. made its broadest challenge to date against open-source software, including Windows rival Linux, claiming that such programs violate 235 Microsoft patents and saying it will seek license fees.

The world's largest software maker said that various open-source packages violate patents it holds in areas related to graphical user interface, e-mail programs and other technology.

"The real question is not whether there exist substantial patent infringement issues, but what to do about them," Horacio Gutierrez, Microsoft vice president of intellectual property and licensing, said in a statement.

Microsoft sent the statement to reporters on Monday via e-mail, detailing a stance that it disclosed in an article that appeared in Fortune Magazine.

Linux is the most popular type of open-source software, or computer programs that developers can download and modify as long as they share those changes with the public. The packages, which are often available for free, have gained in popularity over the past decade, taking market share away from Microsoft.

While dozens of versions of the Linux operating system are available for free download, Red Hat Inc. and Novell Inc. develop and sell versions of Linux used to run servers and mainframe computers in corporate data centers.

Last year, Microsoft signed a marketing, technology and patent agreement with Novell, saying it wants to enter into similar pacts with other open-source software companies.

That agreement, which includes a clause that Microsoft will not sue Novell's Linux customers, incensed the community of open-source software developers, because they said Microsoft would use it to back claims that its intellectual property is being violated by code in Linux and other open source software.

Eben Moglen, founding director of the Software Freedom Law Center, called on Microsoft to disclose the patent numbers of the 235 patents it believes are being infringed upon so that the free software community can evaluate its claims.

"They should name their patents," Moglen said. "They should put up or shut up."

The Software Freedom Law Center is a nonprofit legal services organization that provides free legal aid to developers of open-source software.

Officials with Red Hat couldn't be reached for comment.

Novell Chief Executive and President Ronald Hovsepian said Linux did not infringe on any patents. He told the Reuters Global Technology, Media and Telecoms Summit in New York on Tuesday that the deal with Microsoft was focused on not suing each other's customers.

Novell had not endorsed Microsoft's patent claims, he commented. "We just both agreed to disagree," he said.

In its statement on the issue, Microsoft played up its partnership with Novell, saying that customers worried about intellectual property claims should buy their Linux software from Novell.