He said Thong Lor police had been informed about suspicious activities at a three-storey house in Soi Sukhumvit 31 and raided it on Tuesday afternoon.
Neighbours had reported there were many people living in the house, and they never seemed to leave or go outside. Food was delivered to the house once a day, at lunchtime. The windows were boarded up.
Police obtained a search warrant to get into the building.
The raiding party police found 14 Chinese men and seven Chinese women on the second floor of the house, which was set up as a call-centre with telephones, communication radios and internet routers.
Some of the suspects were busy working the phones and others were at the radio communication set up. Police said they appeared to be passing themselves off as bank officers or police. Many documents in Chinese were also found at the house.
The suspects did not have their passports with them. Police suspect their documents were held by another person who had already fled.
Pol Lt Gen Kamronwat said the 21 suspects did not talk much. Police would work with immigration officials to expand the investigation. It was believed it was only one of several such call-centres set up in Bangkok by the scam gang.
The call-centre scam has been operating for some time and targets Chinese people, with estimated losses already around 100 million baht.