Mr Dart said the site was the largest source of prostitution in the nation
A sheriff in Illinois is suing the online ad site Craigslist, accusing its owners of knowingly promoting prostitution in the US.
Cook County Sheriff Thomas Dart said the site was failing to block offers to trade sex for money.
He wants a federal judge to shut down the site's erotic services section.
In a statement, Craigslist said it had not seen the lawsuit but that it co-operated with police daily to prevent misuse of the site.
"Craigslist is an extremely unwise choice for those intent on committing crimes, since criminals inevitably leave an electronic trail to themselves that law enforcement officers will follow," a spokeswoman said in an emailed statement.
"Misuse of the site is exceptionally rare compared to how much the site is used for legal purposes. Regardless, any misuse of the site is not tolerated on Craigslist," she said.
Speaking at a news conference, Mr Dart called the site "the single largest source of prostitution in the nation".
He said Craigslist's owners "catered their site so it facilitates [prostitution], where you can actually and more specifically and quickly get to what you want".
"Missing children, runaways, abused women and women trafficked in from foreign countries are routinely forced to have sex with strangers because they're being pimped on Craigslist," he said.
"How is that different than somebody who's aggressively and actively working with a pimp to try to get the word out about the women working for him?" he added.
Last year, the San Francisco-based website settled a nationwide lawsuit by promising to enact new rules to crack down on prostitution.
Mr Dart said his officers had seen no change in the number or type of postings in the erotic services section since then.
However, the site's spokeswoman said it was actively working to stop people from circumventing its defences against illegal activity.