PHILADELPHIA  —  Congress must try again if it wants the federal government to police the Internet on behalf of the nation's children.

A federal judge on Thursday blocked a 1998 federal law that makes it a crime for commercial Web site operators to let children access "harmful" material.

The judge said parents can protect their children through software filters and other less restrictive means that do not limit the rights of others to free speech.

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"Perhaps we do the minors of this country harm if First Amendment protections, which they will with age inherit fully, are chipped away in the name of their protection," wrote Senior U.S. District Judge Lowell Reed Jr., who presided over a four-week trial on the issue last fall.

The law would criminalize Web sites that allow children to access material deemed "harmful to minors" by "contemporary community standards." The sites would be expected to require a credit card number or other proof of age. Penalties include a $50,000 fine and up to six months in prison.

Sexual health sites, the online magazine Salon.com and other Web publishers backed by the American Civil Liberties Union challenged the law. They argued that the Child Online Protection Act was unconstitutionally vague and would have a chilling effect on speech.

The U.S. Supreme Court upheld a temporary injunction in 2004 on grounds the law was likely to be struck down and was perhaps outdated.

Critics have noted, for example, that the law does not address e-mail, streaming video or social networking sites. Technology experts say parents now have more serious concerns about Internet safety, such as the threat of online predators.

Discovery in the case sparked a legal firestorm last year when Google challenged a Justice Department subpoena over what kind of information people seek online. Justice lawyers had asked Google to turn over 1 million random Web addresses and a week's worth of Google search queries.

A judge sharply limited the scope of the subpoena, which Google had fought on trade secret, not privacy, grounds.

To defend the nine-year-old Child Online Protection Act, government lawyers attacked software filters as burdensome and less effective, even though they have previously defended their use in public schools and libraries.

"It is not reasonable for the government to expect all parents to shoulder the burden to cut off every possible source of adult content for their children, rather than the government's addressing the problem at its source," a government attorney, Peter D. Keisler, argued in a post-trial brief.

The plaintiffs argued that filters work best because they let parents set limits based on their own values and their child's age.

The law addresses material accessed by children under 17, but only applies to content hosted in the United States.

The Web sites who challenged the law said fear of prosecution might lead them to shut down or move their operations offshore, beyond the reach of the U.S. law. They also said the Justice Department could do more to enforce obscenity laws already on the books.

The 1998 law followed Congress' unsuccessful 1996 effort to ban online pornography. The Supreme Court in 1997 deemed key portions of that law unconstitutional because it was too vague and trampled on adults' rights.

The newer law narrowed the restrictions to commercial Web sites and defined indecency more specifically.

In 2000, Congress passed a law requiring schools and libraries to use software filters if they receive certain federal funds. The high court upheld that law in 2003.