PHILADELPHIA — A Pennsylvania law requiring Internet service providers to block Web sites containing child pornography is unconstitutional and cannot be enforced, a federal judge ruled today.

Enacted in 2002, the law gave Pennsylvania's attorney general the power to order companies such as America Online to block customers from viewing Web sites that had been identified by the state as containing illegal content.

No one challenged the state's right to stop the distribution of child porn, but lawyers for the Center for Democracy & Technology and the American Civil Liberties Union had argued that the technology used to filter out those Web sites was clumsy and produced unintended consequences.

Over two years, the groups said, ISPs 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, but were part of the same Internet cluster as the offending site.

Lawyers for the state said the technology exists for ISPs to block selectively, and said the state's hands should not be tied simply because Internet access companies were reluctant to upgrade their systems.

"Based on the evidence presented by the parties at trial, the Court concludes that, with the current state of technology, the Act cannot be implemented without excessive blocking of innocent speech in violation of the First Amendment," U.S. District Judge Jan E. DuBois wrote.

A costly piece of blocking hardware, called a proxy, can shut down individual sites. However, experts said that such technology comes at a price that would force small ISPs out of business, and large ones like AOL to spend tens of millions of dollars — all for a weapon effective only until the peddlers of online kiddie porn change tactics.