SHANGHAI—China’s securities regulator has launched a probe into Citic Securities Co. for suspected violations of securities rules, escalating a crackdown on the country’s largest stockbroker at the center of a broad campaign to clean up the financial sector following the summer’s stock-market rout.

In a statement to the Shanghai Stock Exchange, Citic Securities said that it has received notification from the China Securities Regulatory Commission regarding the investigation, without offering further details. The Beijing-based brokerage said it will cooperate with the authorities and that the firm’s operations remain normal.

The latest move by the Chinese authorities marks a significant shift in the nature of its probe into the brokerage, which has been at the forefront of Beijing’s effort to modernize its underdeveloped capital market and globalize the reach of its state-owned financial behemoths over the past decades.