US STOCKS-Wall St ends sharply lower, S&P below key support level

* Indexes off: Dow 1.5 pct, S&P 1.6 pct, Nasdaq 1.9 pct

By Ryan Vlastelica

NEW YORK, Sept 25 (Reuters) - U.S. stocks ended with sharp losses on Thursday, with the S&P 500 suffering its biggest one-day decline since July, as Apple (NasdaqGS: AAPL - news) tumbled and the dollar rose to a four-year high.

The day's decline was broad, with all ten primary S&P 500 sectors lower on the day and most down more than 1 percent. About 80 percent of stocks traded on both the New York Stock Exchange and Nasdaq ended lower.

The S&P has dropped for four of the past five sessions and it closed below its 50-day moving average for the first time since Aug 15. That level had previously served as support, and a protracted period underneath it could signal further losses ahead.

The losses came on the continued strength of the dollar , which rose 0.2 percent against a basket of major currencies. It is up 6.8 percent for the quarter, its biggest quarterly increase in six years.

With the day's loss, the S&P is about 2.3 percent below a record close hit earlier this month, meaning it remains far from correction territory - defined as a 10 percent drop from a peak. The S&P hasn't had a correction since 2012, with investors using market declines as buying opportunities, a trend that may yet hold here.

"The rising dollar is a concern, as it could be a headwind for large corporate earnings, but the market looks a lot more attractive than it did just a few days ago," said John Schmitz, portfolio manager at Bahl & Gaynor in Cincinnati. "The selloff could offer an opportunity."

The Dow Jones industrial average fell 264.26 points, or 1.5 percent, to 16,945.80, the S&P 500 lost 32.31 points, or 1.6 percent, to 1,965.99 and the Nasdaq Composite dropped 88.47 points, or 1.94 percent, to 4,466.75.

Apple Inc suffered one of its biggest daily declines of the year, breaking under key technical levels after the tech giant withdrew an update to its new operation system, which some users complained contained numerous glitches.

The stock fell 3.8 percent to $97.87 and was the biggest weight on the S&P, comprising 2.6 points of the index's 32.3 point drop. Apple closed under its 50-day moving average for the first time since April 23, and moved on volume of almost 100 million shares, well above its 50-day average of 56.46 million.

Thursday's losses were so pronounced that no S&P 500 component rose more than 1 percent on the day. The biggest gainer was Motorola Solutions Inc (NYSE: MSI - news) , which added 0.7 percent to $62.24 after it said it would reduce its pension plan liability by $4.2 billion.

Micron Technology Inc (NasdaqGS: MU - news) fell 3.4 percent to $30.61 after the market closed after reporting its fourth-quarter results. Nike Inc (Sao Paolo: NIKE34.SA - news) rose 4.1 percent in after-hours trading after its results.

About 6.3 billion shares traded on all U.S. platforms, according to BATS exchange data, above the month-to-date average of about 6.02 billion. (Editing by Nick Zieminski)