The website Backpage.com sued the state Monday, saying a new law that would require classified-advertising companies to verify the ages of people in sex-related ads is invalid, even if it has a laudable goal.

Gov. Chris Gregoire signed the bill into law this year in an effort to cut down on child-sex trafficking. It allows for the criminal prosecution of classified-ad company representatives who publish or cause publication of sex-related ads peddling children.

Proof of a good-faith attempt to verify the age of the advertised person is considered a defense under the law.

Backpage operates a robust online clearinghouse for escorts, and it's a primary target of the law, which is due to take effect Thursday. The company, owned by Village Voice Media, has come under scrutiny from authorities for allegations that it is used to promote child prostitution.

It sued in U.S. District Court in Seattle to block the law from being enforced pending a judge's decision on whether it should be struck down.

The law is written so expansively, it would apply not just to classified-ad companies, but to dating sites, blogs, chat rooms and social-networking sites, the company argues.

"This means that every service provider — no matter where headquartered or operated — must review each and every piece of third-party content posted on or through its service to determine whether it is an 'implicit' ad for a commercial sex act in Washington, and whether it includes a depiction of a person, and, if so, must obtain and maintain a record of the person's ID," the complaint says. "These obligations would bring the practice of hosting third-party content to a grinding halt."

Backpage argues that the law is trumped by the federal Communications Decency Act, which says online service providers are not responsible for the content of ads placed by third parties. Backpage also says Washington's law is unconstitutionally vague, infringes on First Amendment rights and attempts to regulate activity outside of the state.

Backpage lawyer Liz McDougall said it is "an online industry leader in working cooperatively with law enforcement to identify, arrest and prosecute human traffickers," and that the state law would return criminal conduct underground, where it's harder to track.

"The trafficking of children for sex is an abomination," she said in a written statement. "I believe aggressive improvements in technology and close collaboration between the online service community, law enforcement and (nongovernmental organizations) is the best approach to fighting human trafficking."

Village Voice Media owns 13 alternative weeklies, including Seattle Weekly, which already requires ID from those depicted in sex-related ads.