The eight women visited Long Island this summer along with vacationing families and other business travelers, staying in hotels and motels in commercial strips in middle-class suburbs like East Garden City, Hicksville and Woodbury. Their ages ranged from 20 to 32.

Three had come all the way from the San Francisco Bay area, one from Miami. Two lived less than 60 miles away, in Newark and Elizabeth, N.J. and two even closer, in Brooklyn.

All eight were arrested on prostitution charges here, snared in a new sting operation by the Nassau County police that focuses on Craigslist.org, the ubiquitous Web site best known for its employment and for-sale advertisements but which law enforcement officials say is increasingly also used to trade sex for money.

Nassau County has made more than 70 arrests since it began focusing on Craigslist last year, one of numerous crackdowns by vice squads from Hawaii to New Hampshire that have lately been monitoring the Web site closely, sometimes placing decoy ads to catch would-be customers.

“Craigslist has become the high-tech 42nd Street, where much of the solicitation takes place now,” said Richard McGuire, Nassau’s assistant chief of detectives. “Technology has worked its way into every profession, including the oldest.”

Augmenting traditional surveillance of street walkers, massage parlors, brothels and escort services, investigators are now hunching over computer screens to scroll through provocative cyber-ads in search of solicitors.

In July raids, the sheriff of Cook County, Ill., rounded up 43 women working on the streets — and 60 who advertised on Craigslist. In Seattle, a covert police ad on Craigslist in November resulted in the arrests of 71 men, including a bank officer, a construction worker and a surgeon.

And in Jacksonville, Fla., a single ad the police posted for three days in August netted 33 men, among them a teacher and a firefighter. “We got hundreds and hundreds and hundreds of hits” in phone calls and e-mail messages, said John P. Hartley, the assistant chief sheriff there.

Sex and the Internet have been intertwined almost since the first Web site, but the authorities say that prostitution is flourishing online as never before. And while prostitutes also advertise on other sites, the police here and across the country say Craigslist is by far the favorite. On one recent day, for example, some 9,000 listings were added to the site’s “Erotic Services” category in the New York region alone: Most offered massage and escorts, often hinting at more.

Law enforcement officials have accused Craigslist of enabling prostitution. But the company’s president, Jim Buckmaster, said its 24-member staff cannot patrol the multitude of constantly changing listings — some 20 million per month — and counts on viewers to flag objectionable ads, which are promptly removed.

“We do not want illegal activity on the site,” he said. Asked whether the company supported the police’s placing decoy ads on Craigslist, Mr. Buckmaster said: “We don’t comment on the specifics” of law enforcement.

Craig Newmark, the site’s founder and chairman, deferred all questions to Mr. Buckmaster.

The police have also occasionally turned to Craigslist to trace stolen goods offered for sale or make drug arrests. In June, in Nassau, spotting code words like “snow” or “skiing” to refer to cocaine, they set up a sting with an undercover officer to arrest a man who advertised cocaine for sex.

Experts say that under the federal Communications Decency Act of 1996, the ads are legal and Web site owners are exempt from responsibility for content posted by users. Craigslist, for example, last fall won dismissal of a suit that alleged housing discrimination in ads posted on its Web site. “You hold the speaker liable, not the soapbox,” explained Kurt B. Opsahl, senior staff attorney for the Electronic Frontier Foundation, a digital civil liberties group based in San Francisco.

While Mr. Buckmaster said Craigslist was no different from old-media publications that have long carried sex-oriented ads, law enforcement officials say its scope and format are especially useful to the sex industry. With listings for some 450 cities around the world, Craigslist claims to have 25 million users and 8 billion page views a month. Posting advertisements, except those in the employment and some housing categories, is free, as is responding to them by e-mail.

“The Internet has allowed people to make contact in a way not possible before,” said Ronald Weitzer, a sociology professor at George Washington University and a researcher on prostitution. “Ten years ago this was not happening at all.”

As Nassau’s district attorney, Kathleen Rice, said of Craigslist: “It’s as easy as it gets.”