The online classified advertising site Backpage.com abruptly shut its "adult" section in the U.S. on Monday, yielding to a campaign by some state and federal government officials to close a service they contend promotes prostitution and human trafficking.

The unexpected move came on the eve of a hearing convened by a U.S. Senate subcommittee at which Backpage executives had been ordered to testify. In a letter to the subcommittee that rejects the legitimacy of the hearing, Backpage attorneys said the executives would appear but would not testify.

"The decision of Backpage.com today to remove its Adult section in the United States will no doubt be heralded as a victory by those seeking to shutter the site, but it should be understood for what it is: an accumulation of acts of government censorship using extra-legal tactics," Backpage.com said in a statement. "

Senators Rob Portman and Claire McCaskill, however, said their subcommittee found Backpage had been far more complicit in sex trafficking than previously known. "Backpage's response wasn't to deny what we said. It was to shut down their site," they said in a statement. "That's not 'censorship' – it's validation of our findings."

The move by Backpage also came on the heels of a criminal action in California, where attorney general Kamala Harris filed charges of pimping and money-laundering against Backpage CEO Carl Ferrer and the company's controlling shareholders, Michael Lacey and James Larkin.

Those charges were tossed not long after, however, as a California judge ruled the site is protected by the Communications Decency Act.

Earlier in the day, the U.S. Supreme Court said it wouldn't hear an appeal from three sex trafficking victims who accuse the site of helping to promote the exploitation of children.

The justices left in place a lower court ruling that said federal law shields Backpage from liability because the site is just hosting content created by people who use it.

Replaced, mirrored Craigslist

The women say they were sold as prostitutes in Massachusetts and Rhode Island through advertisements for escort services on the site when they were as young as 15.

A federal judge threw out the lawsuit and the federal appeals court in Boston upheld that ruling.

Backpage Sex Trafficking

Michael Lacey, shown in an Oct. 12, 2016 booking photo, is a former newspaper editor and then Village Voice owner who helped found Backpage.com, the very profitable but highly controversial website. (Sacramento County Sheriff's Office via AP)

Backpage has also faced a legal battle in Washington that went all the way to the state supreme court there, and by mid-2015 the three big credit card companies announced they would no longer allow credit card purchases for adult ads on the site.

Like the decision by Craigslist to remove its adult category in 2010, Monday's announcement is the culmination of years of effort by government at various levels to exert pressure on Backpage.com and to make it too costly to continue," it said.

Lacey and Larkin had decades of experiences in journalism, at several alternative press papers. They eventually came to own the Village Voice in New York in 2005, but had helped originate Backpage, along with Ferrer, in response to Craigslist's dominance cutting into their advertising revenue.

As Craigslist's woes reached their peak, Backpage experienced rapid growth, and Lacey and Larkin sold all their media properties in 2012 to concentrate on the lucrative ad business. Ferrer, based out of the Netherlands, was named CEO two years later.

The company has vowed to continue its legal battles, which have become an important test for the entire internet industry of whether online platforms can be held liable for the content posted on their sites.

Backpage.com also cited praise from law enforcement agencies and child-protection organizations who said the site had been helpful in rooting out human trafficking.

The site's adult sections in local Canada webpages are still accessible.

Internet Sex Trafficking

In this Oct. 21, 2014 file photo, people opposed to child sex trafficking rally outside of the the state supreme Court in Olympia, Wash., one of a number of states in which Backpage has faced legal battles. (Rachel La Corte/The Associated Press)

With files from CBC News and The Associated Press