SEATTLE — The website Backpage.com and a nonprofit group that runs a popular archive of Internet sites asked a federal judge on Friday to block a Washington state law that would require classified advertising companies to verify the ages of people in sex-related advertisements.

Gov. Chris Gregoire signed the law, which passed the Legislature unanimously, to cut down on child sex trafficking. Officials hope it will become a national model for dealing with the pernicious problem; they say an estimated 100,000 juveniles are victimized by child prostitution in the U.S. every year.

The law allows for the criminal prosecution of anyone who knowingly publishes or causes the publication of sex-related ads depicting children, unless they can show they made a good-faith effort to confirm that the person advertised was not a juvenile.

Backpage, which is owned by Village Voice Media and makes millions of dollars a year operating an online clearinghouse for escorts, is a main target of the law and has come under pressure from public officials around the country to shut down the ads.

In a room packed with middle school teachers who were at the Seattle courthouse for a civics course, Backpage lawyer Jim Grant asked U.S. District Judge Ricardo S. Martinez to issue an injunction preventing the law from taking effect.

“Exploitation of a minor, sex trafficking, human trafficking of any kind is abhorrent,” Grant said. “No one in the courtroom disputes that.”

But, he added, “Backpage doesn’t believe it’s an effective way to confront the problem and doesn’t believe it’s consistent with free-speech principles.”

Grant said the law will backfire by driving such advertisements underground or to websites based overseas, where police will have a tougher time investigating them.

Backpage also argues that the law is pre-empted by section 230 of the federal Communication Decency Act, which protects interactive online services from liability for material posted by other people, and which expressly blocks “any state or local law that is inconsistent with this section.”

Backpage was joined in its request by The Internet Archive, a nonprofit organization that runs the “Wayback Machine,” which preserves content that has appeared on the Internet — currently more than 150 billion web pages. The Internet Archive says it could be liable under the law because it maintains versions of websites that might advertise child escorts.

However, other digital-rights organizations, including Free Press and the New America Foundation’s Open Technology Institute, criticized Backpage’s position as unethical.

“Village Voice Media needs to remove human trafficking from its business model,” Timothy Karr, senior director of Free Press, said in a written statement.

The plaintiffs argue that the law could also extend to dating sites, blogs and social media sites such as Facebook, should such escort advertisements migrate there.

Lawyers for Washington state and for its county prosecutors, who are named as defendants, insisted that the law is not pre-empted by the Communications Decency Act because it’s consistent with the purposes of that act. Furthermore, argued David Eldred of the King County Prosecutor’s Office, the law also applies to print advertisements, which would not be covered by the act.

If Washington’s law is constitutional in some cases — as with requiring proof-of-age for print advertisements — it should not be struck down as unconstitutional, Eldred said. And because Backpage has not been prosecuted under the law, he argued, it lacks standing to challenge the law in court — something the company disputes, given that the Legislature identified it as a target when the law was passed.

In court documents, a Seattle police detective provided an example of why the state says the law is needed. On June 7, he wrote, Backpage received a tip that a girl advertised on its site might be underage. The company forwarded the tip to the National Center for Missing and Exploited Children, which relayed it to the FBI, which relayed it to local law enforcement.

Four days later, the girl, 15, was arrested in the Seattle suburb of Federal Way, along with a 29-year-old man who agreed to pay her for sex. That same day, the Seattle detective found another advertisement, for the same girl, using the same photographs and contact phone number, posted under a section for escorts in Wenatchee, east of the Cascade Mountains.

The detective contacted Backpage to have the ad removed. Backpage complied, the detective said, but her ad was soon reposted — more than a half-dozen times. Eventually she was arrested again.

“The measures in place are insufficient,” Eldred said.

Martinez said he expected to rule by the end of next week.

“The proliferation of these types of ads on all of these different sites is very troubling,” he said. “It’s a fascinating argument from a legal perspective, but the sexual exploitation of children on any level is a real human problem.”