Police have raided multiple sites related to Mergeplus in Seoul in an open probe into why the online discount app operator decided to suddenly and sharply reduce its prepaid discount service.

The Seoul Metropolitan Police Agency's financial crime unit searched five locations from Wednesday morning, including Mergeplus headquarters.

A travel ban has been imposed on three individuals, including the president of Mergeplus, as they are booked on charges of violating the Electronic Financial Transactions Act.

Police are looking into suspicions the discount coupon app conducted business without proper registration.

Mergepoint gained huge popularity as an app where consumers can buy points at a 20-percent discount and use them at a variety of places.

But following a registration request by authorities earlier this month, the firm abruptly changed the terms and limited the use of points, leading to a torrent of consumer complaints and calls for refunds. 

Mergeplus submitted partial documents necessary for the registration process, but only after financial authorities requested a police investigation.