These lenders don’t require credit scores or visits to a bank. But they charge high costs over a brief period. They also require access to a borrower’s phone, siphoning up contacts, photos, text messages, even battery percentage.

Then they bombard borrowers and their social circles with pleas, threats and sometimes fake legal documents threatening dire consequences for nonpayment.

In conservative, tightly knit communities, such loss of honor can be devastating.

“If I am labeled a fraud in front of everyone, my self-respect is gone, my honor is gone,” Kiran Kumar, a 28-year-old cement salesman, said in an interview. “What is left?”

Mr. Kumar initially borrowed about $40 from a lender through an online app to supplement his $200-a-month salary. But he couldn’t pay the mounting fees and interest, so he borrowed from others. Eventually, he owed roughly $4,000.

One morning, he said, the harassing calls began soon after sunrise, with the lenders threatening to make his problems public. Mr. Kumar recalls remaining in bed and, for hours, thinking about how he was going to end his life.

The authorities in India are increasingly worried that many more victims like Mr. Kumar may be out there.

The investigations are raising alarms in India over the vulnerability of a population of 1.3 billion people who are still getting accustomed to digital payments.

The apps being used to take advantage of Indians also speak to the global nature of online fraud. Many of the companies use techniques that flourished in China two years ago before the authorities there shut them down.

In India, one police investigation alone in the city of Hyderabad has mapped out about 14 million transactions across the country worth $3 billion over about six months. India’s central bank and national authorities are now investigating.

“It is becoming difficult for us to count the zeros,” said Avinash Mohanty, the joint commissioner of police in Hyderabad.