In a case that may help define the boundaries of free speech, the battle over Internet porn filters at the local library is headed to federal court.

A three-judge panel in U.S. District Court in Philadelphia will hear arguments starting Monday on whether the porn-filtering software mandated by the Children's Internet Protection Act of 2000 is an imperfect technology that can inadvertently block access to important information on health, sexuality and social issues.

"Instead of relying on filtering technology, we should be educating children to develop an internal filter that serves them throughout their lives," Judith Krug of the American Library Association, one of the main plaintiffs, said. "It's not only learning the difference between right and wrong but how to use information wisely ... there are no quick fixes."

The lead plaintiffs are the American Library Association and the Multnomah County Library in Portland, Ore., which wants to offer patrons a choice between filtered and unfiltered Internet access.

Under the law, any library that receives federal technology funding must have "a policy of Internet safety for minors that includes the operation of a technology protection measure with respect to any of its computers with Internet access that protects against ... visual depictions that are obscene, child pornography or harmful to minors." Libraries that don't comply by July 2002 would lose federal technology funding.

Filtering software commonly works by blocking access according to the originating Web site. The filter prevents users from going to Web pages on a "denial list" of unapproved sites. Other filters reject e-mail and chat room messages if they find an objectionable word.

But the plaintiffs said the filters are far from perfect. Krug cited examples of filters blocking Web sites of House Majority Leader Dick Armey and golfer Fred Couples, as well as American Indian groups because of references to peyote -- a plant used in native religious ceremonies but banned in many states for its hallucinogenic properties.

Krug and other opponents also maintain that certain filtering software protocols have been set up with politically conservative or religious biases and others are closely affiliated with religious organizations, which potentially poses a conflict when used in a public library.

Nancy Willard, a research associate at the University of Oregon's College of Education who recently released a report on the issue, said she was "blown away" after discovering that some filtering software blocks sites containing references to witchcraft, homosexuality and anti-government groups.

"It places into question the constitutionality of any public institution adopting the use of filtering technology when they are essentially turning over responsibility to third-party companies to make blocking decisions," she said. "And we have no idea about the basis upon which the blocking decisions are being made."

The government contends that the law does not censor libraries because they can simply decline to accept the federal funding, and that printed pornographic materials are not in a library's collection so there is no reason why online obscenity should be.

Supporters of the law maintain that filtering software has vastly improved since the measure was passed, making fewer mistakes and allowing librarians or other administrators to unblock sites that were blocked in error. Filters also can be depending on whether the user is a child or adult, Miriam Moore of the Family Research Council said.

"They're still not perfect but neither are safety belts, and we use them," she said. "It's a preventive measure."

It takes more than a "tap-on-the-shoulder policy" in libraries to protect children from pornography and predators lurking online, Moore said.

"You'd have to designate a person whose only job would be to watch what people were looking at," she said

Civil-liberties groups have challenged all three federal laws aimed at restricting Internet access to minors. The Supreme Court heard, but has not yet ruled, on one such case in December.

In 1997, it struck down the 1996 Communications Decency Act as unconstitutional. That law made it a crime to put adult-oriented material online where children can find it; the high court ruled that the law was too vague and trampled on adults' rights.

In 1998, the Child Online Protection Act narrowed the restrictions to commercial Web sites and defined indecency more specifically. Sites must collect a credit-card number or other proof of age before allowing Internet users to view material deemed "harmful to minors." The 3rd U.S. Circuit Court of Appeals barred enforcement, saying the standards are so broad and vague that the 1998 law is probably unconstitutional. The Supreme Court is expected to rule this year.

The trial is expected to last at least a week.

The Associated Press contributed to this report.