The ring, operating millions of accounts since 2018, is the largest in Hung Yen yet uncovered, provincial police said Thursday.

Among the arrests, Le Tuan Vinh, 32, Tran Thi Ngan, 30, Hoang Van Tan, 32 and Hoang Van Nham, 28, are considered the masterminds, according to local media.

Upon the bust, police seized several mobile phones, computers, SIM cards and around VND2 billion ($85,963) in cash as evidence. Authorities also froze over 100 bank accounts involved in the case.

The four masterminds managed five level-one agencies via five different gambling websites, with its server located outside Vietnam.

Each website would have around 20 level-one agencies assigned to it, and each level-one agency would have dozens of level-two agencies below, all spread across the country.

These agencies all display gamblers' personal info like phone numbers or social media accounts publicly, so that players can get in touch with each other and perform transactions.

The investigation is ongoing.

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 partake.

Gambling and organizing related gaming activities carry fines of VND10-50 million ($430- 2,150) and jail terms of three to seven years.