Truong Ngoc Tu, 37, the mastermind, and 15 accomplices ran an online gambling game that resembled slot machines, said the crackdown force comprising officers from Hanoi police's crime unit and the Ministry of Public Security's hi-tech crime department.

Users had to download the gambling app on their phones or register directly on a web browser, and deposit cash through a network of agencies to get virtual points for gambling. The points could be encashed at the end of the game.

Tu and his accomplices would instruct gamblers how to deposit and withdraw cash and took a 2 percent commission.

To evade authorities, Tu placed the servers for the games outside Vietnam. All his accomplices are close friends, relatives or IT experts.

The ring began operations in 2018 and has signed up millions of gamblers, the police said.

Gambling remains illegal in Vietnam outside its handful of casinos, and even there Vietnamese need to meet certain financial and other criteria to be allowed to gamble.

Gambling and organizing gambling carries fines of VND10-50 million ($430- 2,150) and jail terms of three to seven years.