Backpage.com, the online classifieds website that has become a target for activists because of its sex ads, is seeking to invalidate a law recently passed in Washington state that would require proof of age for those posting the ads.

The website 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.

Today, a federal judge issued a temporary restraining order against the enforcement of the new law, saying Backpage "has shown a likelihood of success on the merits of its claim." The restraining order remains in effect for at least 14 days.

The law is an attempt to reduce child sex trafficking online, a problem Backpage.com acknowledges and says it is working to stop.

The law, which is set to take effect today, would impose a $10,000 fine and up to year in prison if a website does not make a reasonable attempt to ascertain the age of someone placing an ad for commercial sex.

"Although its ostensible motivation - to prevent the sex trafficking of children - is laudable, the law is not," said Liz McDougall, the attorney for Backpage, which is owned by Village Voice Media.

Backpage's legal filing argues that the Washington bill contradicts a federal law, the Communications Decency Act, which Congress passed in 1996. The act says that Internet service providers or "interactive computer services," like Backpage, are merely hosts and not publishers in the traditional sense. That means that the websites cannot be held liable for material posted on them by a third party. Which is why while the act of prostitution may be illegal, Backpage is not responsible for someone posting an ad for it on their site.

Washington's Attorney General Rob McKenna said some in Congress are exploring how the Communications Decency Act might be changed to protect victims of child sex trafficking.

"A group of bipartisan U.S. senators have taken on this issue in a bold way in the last couple of months," he told Nightline in April. "So we intend to be working with them to make sure there's no federal law that acts as a shield behind which Backpage and other internet publishers can hide."

In the meantime, other states, including New York and New Jersey, are considering legislation similar to the Washington law.