Stocks plunged 9 percent in the last two hours of trading on Thursday before clawing back some of the losses as the escalating debt crisis in Europe stoked fears a new credit crunch was in the making.

The Dow suffered its biggest ever intraday point drop, which may have been caused by an erroneous trade entered by a person at a big Wall Street bank, multiple market sources said.

Indexes recovered some of their losses heading into the close but equities had erased much of their gains for the year to end down just over 3 percent, the biggest fall since April 2009.

"We did not know what a stock was worth today, and that is a serious problem," said Joe Saluzzi of Themis Trading in New Jersey.

Traders around the world were shaken from their beds and told to start trading amid the plunge as investors sought to stem losses in the rapid market sell-off.