The benchmark Shanghai Composite Index SHCOMP, -7.70% plunged 7.7% to close at 3,116.35, posting its biggest daily percentage decline since June 2008 . Prior to Monday's heavy loss, the index was up 4.4% for the month to date, extending gains after finishing 2014 with a sharp 53% advance.
The plunge in mainland China helped to push Hong Kong's benchmark Hang Seng Index HSI, -1.51% down 1.5%, with the Hang Seng China Enterprises — which tracks Hong Kong-listed mainland Chinese companies — off 5%.
The China Securities Regulatory Commission, the nation's top market watchdog, announced Friday that a dozen brokerage firms had been punished for violations of margin-trading rules after a two-week overhaul. Infractions included allowing customers to delay margin repayments by longer than currently allowed. '