The Shanghai Composite dropped nearly 8% as investors reacted to the ruling, which will prevent the brokers from opening new margin trading accounts for three months.

Shares of two affected brokerages -- Citic Securities and Haitong Securities -- quickly lost 10%, the daily limit for stocks trading in Shanghai. Guotai Junan Securities, the third broker, was trading down more than 10% in Hong Kong. The fallout was limited to stocks trading in China. Japan's Nikkei gained nearly 1%, and most regional indices finished higher.

Deng Ge, a spokesman for the China Securities Regulatory Commission, said the brokers were caught rolling over margin trading accounts for a large number of clients "and had been warned," according to Reuters.