HANGZHOU - Police in east China's Zhejiang Province have busted the country's largest underground banking case, which involved transactions totalling 410 billion yuan (64 bln US dollars), the Ministry of Public Security said Thursday.

A total of 100 suspects from eight gangs have been detained since the police launched the investigation in September last year.

Police say the gangs were loosely united by a ringleader, Zhao, who operated dozens of shell companies in Hong Kong. The companies were involved in foreign exchange transactions and money laundry.

In December 2014, arrest warrants were issued for 56 suspects and more than 3,000 bank accounts were frozen. It took almost a year for police to sort through the over 1.3 million suspicious transactions.

One gang boss, Yang, told Xinhua that many customers wanted to avoid China's strict supervision on foreign exchange trade. The gang could earn over 50,000 yuan a day thanks to this.

China has faced an "increasingly arduous and complicated" problem with unapproved financial institutions often used for laundering money obtained from corruption, online gambling and fraud, the ministry said in September.

Police have shut down 37 such banks since August, according to the ministry.

Also on Thursday, police in south China's Guangdong Province said they had busted 10 unapproved banks, responsible for 51.6 billion yuan in illegal transactions, earlier this month.