By Sarah N. Lynch and Lisa Lambert

WASHINGTON (Reuters) - U.S. law enforcement agencies have seized the sex marketplace website Backpage.com as part of an enforcement action by the Federal Bureau of Investigation, according to a posting on the Backpage website on Friday.

Groups and political leaders working to end forced prostitution and child exploitation celebrated the shutdown of Backpage, a massive ad marketplace that is primarily used to sell sex. But some internet and free speech advocates warned the action could lead to harsh federal limits on expression and the press.

The website posting said U.S. attorneys in Arizona and California, as well as the Justice Department's section on child exploitation and obscenity and the California and Texas attorneys general had helped shut down the website.

The Justice Department said late on Friday that a court in Arizona ruled the case remains sealed, which puts any legal information under wraps. In the posting about the seizure, the department had originally said more information would be made public on Friday evening.

A Phoenix FBI official said that there was "law enforcement activity" at the Sedona, Arizona home of Michael Lacey, one of the founders of Backpage, but referred further inquiries to the Justice Department.

Reuters was unable to reach representatives of Backpage for comment.

"Today, Backpage was shutdown. It's a huge step. Now no child will be sold for sex through this website," tweeted Senator Heidi Heitkamp.

Heitkamp helped draft legislation passed by the Senate last month that makes it easier for state prosecutors and sex-trafficking victims to sue social media networks, advertisers and others that fail to keep sex trafficking and exploitative materials off their platforms..

President Donald Trump will sign the bill into law next week, said Heitkamp. The legislation, featured prominently in the popular Netflix documentary "I am Jane Doe," amends the Communications Decency Act, which has shielded website operators from state criminal charges or civil liability if they facilitate sex ads or prostitution.

"Shutting down the largest online U.S. marketplace for sex trafficking will dramatically reduce the profitability of forcing people into the commercial sex trade, at least in the short term," said Bradley Myles, chief executive of Polaris, an international anti-slavery group that runs the National Human Trafficking Hotline.

There would be "a dramatic shift in the marketplace starting tonight," he added.

Backpage and advocacy groups say the ads are free speech protected by the U.S. Constitution.

Backpage has affiliates across the country and around the world, and by 2014 brought in annual revenue of $135 million, the New York Times has reported.

The Supreme Court in January 2017 refused to consider reviving a lawsuit against Backpage that was filed by three young women, who accused it of facilitating their forced prostitution.

The National Center for Missing and Exploited Children has told Congress that nearly three quarters of the cases submitted to the center relate to ads posted on the site. The state of California has said that 90 percent of the site's income were attributable to "adult ads."

In 2016, Texas and California authorities raided the company's Dallas headquarters and arrested chief executive Carl Ferrer and other former company executives on pimping-related charges. The judge in the case ruled the website was protected by the First Amendment, and it was not liable for the speech of third parties.

(Reporting by Sarah Lynch and Lisa Lambert, Additional reporting by David Schwartz in Phoenix and Justin Mitchell in Washington, Editing by Toni Reinhold and Rosalba O'Brien)