MUMBAI, India — You know the messages. They pop up on your computer screen with ominous warnings like, “Your computer has been infected with a virus. Call our toll-free number immediately for help.”

Often they look like alerts from Microsoft, Apple or Symantec. Sometimes the warning comes in a phone call.

Most people ignore these entreaties, which are invariably scams. But one in five recipients actually talks to the fake tech-support centers, and 6 percent ultimately pay the operators to “fix” the nonexistent problem, according to recent consumer surveys by Microsoft.

Law enforcement authorities, working with Microsoft, have now traced many of these boiler rooms to New Delhi, India’s capital and a hub of the global call-center industry. On Tuesday and Wednesday, police from two Delhi suburbs raided 16 fake tech-support centers and arrested about three dozen people. Last month, the Delhi authorities arrested 24 people in similar raids on 10 call centers.