GURGAON, India — As the anesthetic wore off, Naseem Mohammed said, he felt an acute pain in the lower left side of his abdomen. Fighting drowsiness, he fumbled beneath the unfamiliar folds of a green medical gown and traced his fingers over a bandage attached with surgical tape. An armed guard by the door told him that his kidney had been removed.

Mohammed was the last of about 500 Indians whose kidneys were removed by a team of doctors running an illegal transplant operation, supplying kidneys to rich Indians and foreigners, police officials said. A few hours after his operation on Thursday, the police raided the clinic and moved him to a government hospital.

Many of the donors were day laborers, like Mohammed, picked up from the streets with the offer of work, driven to a well-equipped private clinic, and duped or forced at gunpoint to undergo operations. Others were bicycle rickshaw drivers and impoverished farmers who were persuaded to sell their organs, which is illegal in India.

Although several kidney rings have been exposed in India in recent years, the police said the scale of this one was unprecedented. Four doctors, five nurses, 20 paramedics, three private hospitals, 10 pathology clinics and five diagnostic centers were involved, Mohinder Lal, the police officer in charge of the investigation, said. "We suspect around 400 or 500 kidney transplants were done by these doctors over the last nine years," Lal, the Gurgaon police commissioner, said.

The case has enthralled India's newspapers. Editorial writers have been particularly incensed by the failure of the police to capture the main doctor, who has many names but was known most recently as Amit Kumar.

He was arrested in 1994 on suspicion of running a kidney transplant racket in Mumbai, but jumped bail, changed his name and set up work again from several clinics hidden in residential apartments in Gurgaon, a prosperous city outside Delhi.

The police raided one of his clinics in 2000, but somehow he was allowed to continue working. Officials neglected to investigate further even after at least one television investigation exposed his work.

On Tuesday, The Times of India called on the government to investigate "the nexus between the organ traders and the police."

Investigators were alerted to the existence of the ring on Thursday by a donor who said the operation had ruined his health.

Apparently tipped off to the raid, Kumar escaped arrest. Only one of the four main doctors implicated has been detained.

The officials suspect that several private hospitals in Delhi and its suburbs were quietly complicit in Kumar's work and treated patients recovering from kidney transplants.

"Due to its scale, we believe more members of the Delhi medical fraternity must have been aware of what was going on," Lal told reporters on Monday.

Lal said a team of criminals he called kidney scouts usually roamed the labor markets in Delhi and cities in Uttar Pradesh, India's poorest state, searching for potential donors. Some prospects were asked outright if they wanted to sell a kidney and were offered $1,000 to $2,500.

A car equipped with testing equipment was often on hand so that potential donors could be checked immediately to see whether their kidneys matched the needs of prospective patients.

Letters and e-mail messages from 48 foreigners inquiring about transplants were discovered in Kumar's office, Lal said. Five foreigners — three from Greece and two Indian-born U.S. citizens — were found in one of the clinics during the raids. The police suspected that they had been about to receive kidney transplants, Lal said, but they were later allowed to return home because there was insufficient evidence to detain them