U.S. stock indexes tumbled about 2% on Monday - the first trading day of the year - after weak Chinese economic data reignited fears of a global slowdown.

Mainland Chinese shares fell 7%, triggering a new circuit breaker that prompted a trading halt, after surveys showed factory activity in the world's second-largest economy shrank sharply in December.

Adding to investors' worries, China's central bank fixed the yuan at a 4-1/2 year low, weakening it against the dollar.

"Those are violent New Year fireworks. That's quite a way to start the day off," said Andre Bakhos, managing director at Janlyn Capital LLC in Bernardsville, New Jersey.

"Right now, the focal point is China, the global economic condition, and the fact that we're coming off a disappointing year on many levels, a frustrating year on many levels, only to walk in and have the (S&P) futures down 35 points," he said.

At 9:36 a.m. ET (1436 GMT), the Dow Jones industrial average was down 349.78 points, or 2.01%, at 17,075.25, the S&P 500 was down 37.67 points, or 1.84%, at 2,006.27 and the Nasdaq Composite index was down 111.77 points, or 2.23%, at 4,895.64.

All 10 major S&P sectors were lower, led by the 2.4% decline in the tech sector.

Apple was the biggest drag on the S&P and Nasdaq, falling 2% to $103.16. The iPhone maker's stock closed 2015 down more than 4%.

Goldman Sachs was down 3.2% at $174.45 and was the biggest drag on the Dow.

Crude oil prices rose after a breakdown in diplomatic ties between Saudi Arabia and Iran raised concerns of supply restrictions, while gold jumped more than 1.5% as investors fled to the safe-haven metal.