US stocks slumped Wednesday on worries about China and Greece in a session that was overshadowed by a lengthy outage at the New York Stock Exchange due to technical problems.

At the closing bell, the Dow Jones Industrial Average was at 17,518.42, down 258.49 points (1.45 percent).

The broad-based S&P 500 fell 34.43 (1.65 percent) to 2,046.91, while the tech-rich Nasdaq Composite Index tumbled 87.70 (1.75 percent) to 4,909.76

The NYSE was closed for more than three hours due to an "internal technical issue" that it said was not caused by hacking.

"We had the NYSE halted but I don't think this was a factor in this decline," said Peter Cardillo, chief market economist at Rockwell Global Capital. "I think this is due to the acceleration of the turmoil in China."