* China factory activity at 6-1/2-yr low in September
* U.S. factory activity stays at 2-yr low in September
* Materials and energy sectors lag
* Indexes down: Dow 0.3 pct, S&P 0.2 pct, Nasdaq 0.1 pct (Updates to late afternoon)
By Caroline Valetkevitch
Sept 23 (Reuters) - U.S. stocks were down slightly in late trading on Wednesday after weak Chinese and U.S. factory data added to global demand worries.
Trading was once again choppy, with the S&P 500 briefly trading higher in the afternoon following comments by Chinese President Xi Jinping that his country was capable of maintaining a relatively high growth rate for a long time.
U.S. manufacturing growth stayed at a two-year low in September, while Chinese factory activity shrank to a 6-1/2 year low in the month.
The S&P materials index was down 1.8 percent, leading the decline for the S&P 500, followed by the energy index, which was down 1 percent.
The CBOE Volatility index, Wall Street's fear gauge, was down 1.7 percent at 22.06, though still above its long-term average of 20.
The volatility in the U.S. stock market has increased recently as investors fret over a China-led global economic slowdown, a concern the Federal Reserve alluded to last week when it left interest rates unchanged.
"I think it is very much technical trading," said Robert Phipps, a director at Per Stirling Capital Management in Austin, Texas.
At 2:51 p.m., the Dow Jones industrial average fell 44.26 points, or 0.27 percent, to 16,286.21, the S&P 500 lost 3.27 points, or 0.17 percent, to 1,939.47 and the Nasdaq Composite dropped 3.83 points, or 0.08 percent, to 4,752.89.
The S&P 500 has seen moves of at least 1 percent in more than 12 sessions since Aug. 20.
First Niagara Financial was up 12.8 percent at $10.12 after Bloomberg reported the regional bank was exploring a sale.
Declining issues outnumbered advancing ones on the NYSE by 1,840 to 1,136, for a 1.62-to-1 ratio on the downside; on the Nasdaq, 1,645 issues fell and 1,109 advanced for a 1.48-to-1 ratio favoring decliners.
The S&P 500 posted no new 52-week highs and 36 lows; the Nasdaq recorded 21 new highs and 124 lows. (Additional reporting by Tanya Agrawal; Editing by Savio D'Souza and Nick Zieminski)