PHILADELPHIA (Reuters) - Pennsylvania state officials pressed ahead with enforcement of a child-pornography law even though they were told it would render thousands of legitimate Web sites inaccessible, officials told a U.S. judge on Thursday.

Internet providers must comply with orders to block child-pornography Web sites under a 2002 state law, an approach that civil liberties groups say is unnecessarily ham-fisted.

State officials have asked Internet providers like Time Warner Inc.'s (TWX.N: Quote, Profile, Research) . America Online to block about 500 Web sites since the law took effect.

But AOL and other Internet providers told investigators they could not block the sites in question without blocking other, unrelated Web sites that share the same online address, state officials testified.

John Burfet Jr., chief counsel for the computer forensics division at the state attorney general's office, said he was not concerned with the techniques used as long as the offending material was blocked.

"We didn't want to be in the business of telling ISPs how to block the Web sites. What mattered was the results," Burfet said. "If ISPs could show the Web sites were inaccessible, we were satisfied."

The Center for Democracy and Technology, a Washington nonprofit group that has sued to overturn the law, said in court filings that up to 600,000 legal Web sites have been inadvertently blocked, including one for a community recreation center in the northeast part of the state.

The Pennsylvania chapter of the American Civil Liberties Union and Internet provider Plantagenet Inc. are plaintiffs in the suit as well.

A spokesman for the state attorney general said the office had not heard many complaints about blocked sites.

"If there were hundreds of thousands of legal Web sites being blocked, as the plaintiffs claim, we believe we would have heard about that," said Sean Connolly, a spokesman for Acting Attorney General Gerald Pappert.

On Wednesday, a computer expert said that those interested in viewing child pornography could use an identity-cloaking service or route their access through an alternate computer called a proxy server to evade the blocks.

"It's a trivial matter to completely circumvent" blocks imposed by Internet providers, said Mitchell Marcus, a computer science professor at the University of Pennsylvania.

U.S. District Judge Jan Dubois said the technical ins and outs of the material at hand posed a challenge.

"I cannot recall a case so complex and with so much data being presented," Dubois said. (Additional reporting by Andy Sullivan in Washington)