China has arrested 75 people accused of engaging in underground banking, the Ministry of Public Security said, as authorities work to curb money laundering and illegal cash transfers.

Authorities also discovered 37 underground banking dens, the ministry said in a statement on its website. Those arrested had dealings totalling more than 240 billion yuan ($38 billion).

China said in August it would launch a three-month crackdown on underground banking. Fears over an economic slowdown and market instability have sparked a wave of capital outflows this year.

Chinese law prohibits people from transferring more than $50,000 out of the country per year.

Underground banking presents an increasingly complex threat to China's financial security, the ministry said in the statement, encompassing issues from financing for drugs and terrorism to tax fraud.