About $36.5m was stolen after US citizens duped into sending money to swindlers in Mumbai posing as US tax officials.

Police in India say they have arrested 70 call centre workers on suspicion of tricking American citizens into sending them money by posing as US tax officials.

A total of 772 workers were arrested on Wednesday in raids on nine fake call centres in a Mumbai suburb, a senior police official told Reuters news agency.

An estimated $36.5 million was extorted from Americans, police said.

Seventy were placed under formal arrest, 630 were released pending questioning over the coming days, and 72 were freed without further investigation.

“The motive was earning money,” said Parag Marere, a deputy commissioner of police. “They were running an illegal process, posing themselves as officers of the [US] Internal Revenue Service.”

Marere said the year-long scam involved running fake call centres that sent voicemail messages telling US nationals to call back because they owed back taxes.

Those who called back and believed the threats would fork out thousands of dollars to “settle” their case, he said. The scam brought in more than $150,000 a day.

Indian police told a court on Thursday that employees applauded when victims transferred large sums electronically to the call centre.

“Employees were aware of the fraud, but since they were getting a good salary, they remained silent,” assistant police commissioner Bharat Shelke said.

The organizers of the scheme were cautious about paper trails, said another police official, Mukund Hatote.

The 70 detainees were driven to court in large vans. Most were young men who walked with heads bowed, their faces obscured by a raised hand or a tied kerchief. Inside, the judge heard arguments from about two dozen defence lawyers who insisted their clients were following orders, and the scheme’s masterminds were still at large.

“I got to know we are doing illegal things within a week after joining,” one defendant told Reuters as he returned to a police van, “but I saw many getting good performance bonuses. So I didn’t leave.”

India is home to a vast number of back-office operations for North American and European companies.

Thousands of call centres in India provide services to these firms, processing everything from utility payments to credit card bills.