Justice Department attorneys, defending a law aimed at keeping online pornography from minors, argued that software filters often block valid sites — on gay rights or sexual health, for example — that teens might seek out.

"Filters are hindering minors from learning about the world around them. That's a huge problem," government lawyer Joel McElvain said Monday. "There may be reasons the teenagers have problems speaking to their parents about these (issues)."

Under the 1998 Child Online Protection Act, commercial Web publishers who fail to keep material "harmful to minors" away from children could face fines and even prison time.

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The law has never taken effect, because of a long-running legal challenge filed by Salon.com, the Philadelphia Gay News and other groups represented by the American Civil Liberties Union.

Closing arguments concluded Monday before Senior U.S. District Judge Lowell Reed Jr., ending four weeks of testimony.

Justice attorneys evoked problems with the software filters even as they had defended their use in a 2000 law requiring schools and libraries to block porn if they receive certain federal funds. The high court upheld that law in 2003.

Opponents say the 1998 law is overly vague and would have a chilling effect on their work. They also say it would not apply to foreign Web operators or streaming video and audio.

The ACLU argues that filters are more effective than legislation, because they let parents set limits based on their own values and their children's ages.

"If you're a parent who doesn't want sexually explicit material slipping through, then set it (the filter) strictly," ACLU lawyer Chris Hansen told the judge Monday.

The U.S. Supreme Court, indicating the government was unlikely to prevail, granted the ACLU a temporary injunction in 2004.

Technology experts not involved in the case have noted that many parents now have more pressing concerns about their children's use of the Internet, such as online predators and the popularity of social-networking sites.

The Justice Department raised eyebrows as it prepared for trial when it demanded reams of privately held information from Google Inc. (GOOG), Yahoo Inc. (YHOO) and other Internet companies for a study on the prevalence of online pornography.

Google fought a subpoena to turn over 1 million sample queries and 1 million Web addresses in its database, citing trade secrets. A judge sharply limited the scope of the subpoena.

The resulting study, conducted by a University of California, Berkeley statistics professor, concluded that about 1 percent of Web sites indexed by Google and Microsoft Corp. are sexually explicit and that about 6 percent of searches yield at least one explicit Web site.

Hansen said filters can be as much as 98 percent effective in blocking explicit material, even if the most stringent filters also "overblock" a large number of acceptable sites.

Congress first tried to regulate online pornography in 1996 with a law that was largely struck down by the Supreme Court the following year.

The 1998 law narrows the restrictions to commercial Web sites and defines objectionable material as obscene or that which offends "contemporary community standards."

The law, signed by then-President Clinton, requires Web site operators to prevent youngsters from seeing material harmful to children by demanding proof of age — such as a credit card number — from computer users.

It would impose a $50,000 fine and six-month prison term on commercial Web site operators that allow minors to view such content.