Taiyuan, a man looks sad in a securities business hall. (China News Service/Wei Liang)

China's securities regulator said on Tuesday that it is investigating possible irregularities in Monday's huge sell-off that gave the benchmark index its worst day in eight years.

Zhang Xiaojun, spokesman for the China Securities Regulatory Commission, said the regulator has tracked stock exchange market monitoring reports and public complaints.

The one-sentence statement came as a sharp sell-off in China's stock market rattled investors and sent the benchmark Shanghai Composite Index down by 8.48 percent on Monday, in the steepest daily drop since Feb. 27, 2007.

Instability continued on Tuesday when the Shanghai index fell 5 percent in the morning, yo-yoed throughout the day, ending down 1.68 percent.