China's benchmark stock index plunged 5.88 percent Monday morning after regulators cracked down on rule violations for margin trading, which has fuelled an extended rally, analysts said.
The benchmark Shanghai Composite Index slumped 198.41 points to 3,178.09.
The Shenzhen Composite Index, which tracks stocks on China's second exchange, lost 3.39 percent, or 50.11 points, to 1,428.36.
The China Securities Regulatory Commission (CSRC), the market regulator, said late on Friday that it had suspended three brokerages from opening new margin trading accounts for customers for three months after rule violations were found in an inspection.
Chinese stocks have been on a powerful rise in recent weeks, triggered by an interest rate cut in November and driven by liquidity and margin trading -- investors using borrowed funds to trade on the markets with only a small portion of money put down as deposit.
The three named brokers -- Citic Securities, Haitong Securities and Guotai Junan Securities -- are among the country's biggest.
"The CSRC's punishment of the three brokerages for rule violations for margin trading business last Friday was a punch to the market," BOC International analyst Shen Jun told AFP.
The securities regulator might also want the market to ease back, following a more than 50 percent rise in the Shanghai index last year, he said.
"If the stock market continues to be red-hot, funds unleashed from monetary easing will flow into the market, so the regulator wants it to take a break."