A federal judge Friday threw out a Pennsylvania law requiring Internet service providers to block Web sites containing child pornography, saying the tools to do so also cause "massive suppression" of constitutionally protected material.

The 2002 law was aimed at forcing companies like America Online (search) to block customers from viewing Web sites with sexually explicit images of children.

No one challenged the state's right to stop child porn (search), which is already illegal under federal law. But lawyers for the Center for Democracy and Technology (search) and the American Civil Liberties Union (search) had argued that the filtering technology used to block such Web sites is too clumsy.

Over two years, the groups said, Internet service providers trying to obey blocking orders were forced to cut access to at least 1.5 million legal Web sites that had nothing to do with child pornography or even legal pornography, but shared Internet addresses with the offending sites.

U.S. District Judge Jan E. DuBois agreed the law could not be enforced without also blocking protected material.

"There is little evidence that the act has reduced the production of child pornography or the child sexual abuse associated with its creation," DuBois wrote. "On the other hand, there is an abundance of evidence that implementation of the act has resulted in massive suppression of speech protected by the First Amendment."

Lawyers for the state had argued that more selective filtering technology exists but that Internet service providers simply do not want to pay for it.

Equipment is indeed available to shut down individual sites, but experts say such costly technology would drive smaller Internet service providers out of business and force larger ones to spend tens of millions on a weapon effective only until the peddlers of kiddie porn change tactics.

The law had called for maximum fines of $30,000 and seven years in prison. Pennsylvania is the only state to pass such a law, though Maryland, New Jersey and Oklahoma have considered similar legislation, said Alan Davidson, associate director of the Center for Democracy and Technology.

"This should send a strong signal that this entire approach to regulating the Internet is flawed," Davidson said.

Sean Connolly, a spokesman for the state attorney general's office, said officials have yet to decide whether to appeal.

"This law was designed to block access to child pornography sites," Connolly said. "We believe it has worked well in Pennsylvania."

At the federal level, the Supreme Court has rebuffed Congress' attempts to ban or restrict adult-oriented Web sites, though it endorsed a law requiring schools and libraries receiving federal funds to use filtering software to block pornography, not just child porn.

Though Pennsylvania law covered only Internet users in the state, Internet service providers typically responded by imposing filters on all their customers.