One of the world's biggest providers of Internet classified advertising abruptly shut down the "adult services" section of its U.S. Web sites this weekend, apparently in response to criticism from prosecutors that it had become a tool for prostitution.

But it was unclear whether Craigslist - an icon of the new media transformation of American society - had closed the site to placate those critics, or to hit back at them, casting itself as the victim of censorship. On its famously bare-bones Web sites, the blue-lettered link for adult services was gone. It had been replaced with a black box, containing one word: "censored."

Craigslist's usually outspoken leaders gave no explanation for their move and no signal as to whether it would be permanent. Last year, the site increased the screening of these ads after authorities in New England said a man had killed one woman and attacked two others he'd contacted through Craigslist.

On Craigslist sites in other nations, the "erotic" sections remained open.

The site's critics greeted the change with praise and wariness.

"They lack either the will or the wherewithal to effectively screen for prostitution ads. Which is why we [said] to them, 'Shut down the site,'" said Connecticut Attorney General Richard Blumenthal (D), one of 18 attorneys general who made that demand in a letter to Craigslist last month. He added, "we hope that their example in doing the right thing will lead others to follow them."

The long-running battle over Craigslist's "adult" or "erotic" ads - which can be thinly veiled as advertisements for a massage or can contain explicit photos and text - illustrates the complexity of policing the Internet. On one side is an iconoclastic company defending what it sees as a major virtue of the Web: the ability to create a self-regulating virtual commons. On the other are prosecutors and anti-prostitution activists who say that the anonymity of the online world can be a great vice.making it easier for people to exploit women and children in the real world

By choosing the word "censored," Craigslist seemed to signal that the battle will continue, said Jason Schultz, a law professor at the University of California at Berkeley.

"Maybe what [Craigslist is] trying to do is raise the other side of the issue, which is that there's serious First Amendment, freedom-of-information issues" on their side, Schultz said. "This seems to me completely adversarial, still."

Craigslist began as an e-mail newsletter in San Francisco in 1995, and grew into a hub for free classified ads that has more than 700 local sites around the world. The company says it has more than 50 million users in the U.S., competing with The Washington Post and other newspapers for classified ads in their circulation areas.

Most of the site's ads can be placed for free. But "adult" ads cost $10 apiece, and there are enough of them to bring in about $36 million in revenue estimated early this year - about a third of the company's total - according to a recent analysis by the Advanced Interactive Media Group.

But both prosecutors and anti-prostitution groups say these ads have become a bazaar for prostitutes and pimps. Instead of taking the risk of meeting customers on the street, the activists say, they can arrange visits in hotel rooms.