China's main stock market index fell more than 5 percent after two of the country's biggest securities firms announced they are under investigation for possible misconduct.

The Shanghai Composite Index on Friday fell 5.2 percent before recovering slightly. The decline was led by CITIC Securities and Guosen Securities, which fell by the full 10 percent daily limit following announcements by both that they were under investigation.

A growing string of Chinese securities executives have been detained or questioned by regulators following a plunge in share prices that began in early June.

The announcements by CITIC and Guosen were the first that brokerages themselves were being investigated.