Needy patients and impoverished donors are coming together via the Web or through middlemen who charge buyers as much as €100,000 for a kidney, plus the costs of the operation and a return plane ticket.

While reliable statistics are difficult to come by, 15,000 to 20,000 kidneys are illegally sold globally each year, according to Organ Watch, a human rights group in Berkeley, California, that tracks the illegal organ trade. The United Nations estimates that 5 to 10 percent of kidney transplants performed each year are the result of organ trafficking.

Nancy Scheper-Hughes, the director of Organs Watch and professor of medical anthropology at the University of California, Berkeley, said the attempt by poor Europeans to sell their organs was reminiscent of the period after the collapse of the Soviet Union when chronic joblessness created a new breed of willing sellers, as well as criminal networks ready to exploit their misery.

Mr. Ratel, the prosecutor, said that until recently, Turkey was the main European hub for trafficking; organ tourists typically come from the United States, Britain, France, Israel, Italy and Germany. He said the most popular Internet portals for organ sales had Russian domain names. Often, the donors never see a penny, he said.

In Doljevac, a poor municipality of 19,000 people in southern Serbia, whose once flourishing tobacco, slaughterhouse and agriculture industries have fallen on hard times, residents recently tried to register a local agency to sell their organs and blood abroad for profit, but were refused by the government.

Violeta Cavac, a housewife advocating for the network, said the unemployment rate in Doljevac was 50 percent and more than 3,000 people had wanted to participate. Deprived of a legal channel to sell their organs, she said locals were now trying to sell body parts in neighboring Bulgaria or in Kosovo.

“I will sell my kidney, my liver, or do anything necessary to survive,” she said.

She is not alone. Hunched over his computer on a recent day in Kovin, about 40 kilometers from Belgrade, Mr. Mircov showed a reporter his kidney-for-sale ad, which included his blood type and phone number. “Must sell kidney. Blood group A,” the ad said. “My financial situation is very difficult. I lost my job, and I need money for school for my two children.”