Detective police said four people were arrested during a raid in Dhaka's Bashundhara Residential Area on Saturday night.
Police said the arrests involved three Chinese nationals and one Bangladeshi national.
The operation was announced by Additional Commissioner of Police Mohammad Shafiqul Islam at a press briefing on Sunday.
Police alleged that online gambling, fraud and unlawful digital-gateway activity were being run from flats on the fourth and fifth floors of a six-storey building.
Officers said they seized 6,000 SIM cards, 20 digital gateway machines, 14 laptops, 16 mobile phones and Tk209,700 in cash.
Police also listed foreign liquor among the items recovered from the premises.
Shafiqul Islam said the group sent text messages advertising attractive offers, jobs and additional income.
He alleged that recipients who opened links in those messages were induced to send money through different purported offers.
Police said the seized gateway equipment had capacity for between 10,000 and 12,000 SIM cards.
Police alleged that the operators regularly changed device IMEI numbers, making the operation harder to identify.
Shafiqul Islam estimated that the group had moved Tk200 crore to Tk250 crore from Bangladesh each month through online gambling and other illegal means.
Police said legal action and a case had been initiated against the detainees.
The allegations, the claimed scale of financial transfers and any eventual criminal liability remain to be tested through the legal process.