The trigger for the sell-off came from China, where the world's second-largest economy expanded 7.7 percent in the first three months of the year, well below forecasts of 8 percent or better. That news pummeled copper, oil and other commodities. Shares of oil and mining companies fared the worst because China is a huge importer of their products.
Stocks extended losses as explosions rocked the finish line area of the Boston Marathon, but almost all of the decline came before the blasts.
A pile of negative economic reports weighed on stocks. In addition to the concerns about China, a separate report showed weak manufacturing in the Northeast, and a home builders' survey indicated housing activity isn't going to be strong, either, said Steven Ricchiuto, chief economist for Mizuho Securities.
The pullback disrupted the rally that has sent the Dow up 13 percent and the Standard & Poor's 500 index up 11 percent in 2013.
The Dow tumbled 265.86 points to close at 14,599.20. The S&P 500 fell 2.30 percent to 1,552.36. The Nasdaq fell 2.38 percent to 3,216.49.