Stock market endures worst day in 18 months

The U.S. stock market is closing with its worst performance in 18 months, driven lower by another slump in Chinese shares and heavy selling by technical traders.

The Standard & Poor's 500 index dropped 43 points, or 2.1 percent, to close at 2,035. It was the index's biggest percentage decline since February 2014. The Dow Jones industrial average slid 358 points, also 2.1 percent, to 16,990. 

The Nasdaq fell 141 points, or 2.8 percent, to 4,877.

Worries over China were again the catalyst for the losses. The Shanghai Composite dropped 3.4 percent Thursday on heavy selling of energy and property companies.

Strategists and traders say the U.S. rout is also likely tied to programmed selling after the S&P 500 breached one of the most closely watched indicators, its 200-day moving average