SAN FRANCISCO — The online classifieds company Craigslist said on Thursday that it had reached an agreement with 40 state attorneys general and agreed to tame its notoriously unruly "erotic services" listings.

Prostitutes and sex-oriented businesses have long used that section of Craigslist to advertise their services. Along with their ads, they often include pornographic photos.

Early this year, the attorney general of Connecticut, Richard Blumenthal, representing 40 states, sent a letter to Craigslist demanding that it purge the site of such material and better enforce its own rules against illegal activity, including prostitution. The two sides began a series of conversations about what Craigslist could do to prevent such ads from appearing.

"They identified ads that were crossing the line," said Jim Buckmaster, chief executive of Craigslist. "We looked at those ads, we saw their point, and we resolved to see what we could do to get that stuff off the site."

In March, Craigslist created a hurdle for anyone trying to place certain ads. The company began asking its erotic services advertisers to provide a phone number, which an automated system calls. The system reads a series of digits, which the advertiser has to type into a Web page as verification before the ad will appear on the site. Craigslist said that reduced a majority of the illicit material.

Under the broader agreement announced on Thursday, Craigslist is going further, asking that advertisers provide valid identification. It said that it will charge erotic services vendors a small fee for each ad — about $10, Buckmaster said — and require that they use a credit card for the payment. It will donate the money to charities that combat child exploitation and human trafficking. This, theoretically, will let the company confirm not just a phone number but also an identity.

On Wednesday, Craigslist also filed 14 lawsuits in San Francisco against companies that were helping advertisers circumvent the telephone verification system by generating Internet telephone numbers that could be used temporarily and then discarded.

Blumenthal said that the new measures would discourage many sex operators from using Craigslist. "The mere act of authentication will be a very significant deterrent," he said. "There are very few prostitutes who want to be called by Craigslist and asked to give additional identifying information."