Alec Young, a global equity strategist at S&P Capital IQ, said investors weren't expecting Bernanke to say the program could end so quickly, and are adjusting their portfolios in anticipation of higher U.S. interest rates.

"What we're seeing is a pretty significant sea-change in investor strategy," Young said

For much of the year, the stock market rose with barely an interruption. The S&P 500 climbed for seven months straight from November 2012 through May. Investors, fearful of missing out on the rally, pounced on any dips and pushed markets to record highs. On Thursday, those opportunistic buyers were absent. Nobody wanted to stand in the way of the market's slide.

As investors sold stocks, they likely put the proceeds in cash "for fear the deterioration will continue," said Quincy Krosby, a market strategist at Prudential Financial.

The sharp increase in bond yields prompted investors to sell homebuilders, whose business could be hurt if the pace of home buying slows down. Those stocks fell Thursday even though the National Association of Realtors said U.S. sales of previously occupied homes last month topped 5 million at an annual rate for the first time in 3 ½ years.

PulteGroup plunged $1.89, or 9.1 percent, to $18.87. D.R. Horton fell $2.13, also 9.1 percent, to $21.31.

Markets were also unnerved after manufacturing in China slowed at a faster pace this month as demand weakened. That added to concerns about growth in the world's second-largest economy. A monthly purchasing managers index from HSBC fell to a nine-month low of 48.3 in June. Numbers below 50 indicate a contraction.

A big jump in the overnight lending rate in China also unsettled investors, said Brad Reynolds, a financial advisor at LJPR. The rate measures how much banks charge each other to borrow short-term money. The People's Bank of China was forced to pump about 50 billion yuan, about $8 billion, into the Chinese financial system to alleviate the squeeze, Bloomberg News reported.

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