The Washington-based Center for Democracy and Technology (CDT) brought the case together with the American Civil Liberties Union (ACLU) of Pennsylvania and Penn state ISP Plantagenet Inc.
Under the controversial law, the state Attorney General or any county district attorney can unilaterally apply to a local judge for an order declaring that certain internet content may be child pornography, and requiring any ISP serving Pennsylvania to block the content.
The entire court proceeding, say the claimants, occurs with no participation by the targeted ISP or the web site owner, violating the due process and free speech protections of the Constitution.
According to Wired News, an ISP that fails to comply with an order within five days of receiving it faces a maximum fine of $30,000 and a possible prison sentence of seven years.
Over 500 orders have been sent since the law came into force in 2002, with the result, according to Reuters, of more than 600,000 sites being blocked.
According to the suit, because of technical difficulties most ISPs can only comply with the order by blocking a large volume of innocent web content as well as target pages. Often this means blocking access throughout the ISP's network, meaning that the Pennsylvanian orders affect internet users throughout the US.
The Attorney General's office disputes this, with spokesman Sean Connolly telling Reuters:
"If there were hundreds of thousands of legal web sites being blocked, as the plaintiffs claim, we believe we would have heard about that."
In a statement issued when the suit was filed, CDT Associate Director Alan Davidson said, "Child pornography has no place in a civilized society". He continued:
"Unfortunately, Pennsylvania's web blocking law does little to stop child pornography but does a great deal to violate the protections of the First Amendment. Rather than blocking swaths of legal internet content in an effort to do nothing more than hide child pornography, law enforcement should go after child porn at the source [sic] the criminals who create and distribute it."
The CDT fears that if the law is upheld it could set a dangerous precedent of regulating ISPs and other intermediaries without any notice to the publishers who might be affected.
A ruling is expected within a few weeks.
On Friday, the UK-based NCH Children's Charity published a report claiming that child pornography crimes have increased by a factor of 15 since 1988 because of the internet. It warns that video-enabled 3G mobile phone technology will further increase the risks because users with pay-as-you-go handsets will be impossible to trace.