The Dow Jones industrial average struggled for gains in volatile trading, set to post its worst weekly performance since October 2008.

The Dow fell once again in the final hour of trading, down 100 points as of 3:11 p.m. ET; the index has swung nearly 850 points in volatile trading. The S&P 500 and the Nasdaq composite joined the Dow in its trip lower, falling 0.4 percent and 0.3 percent respectively.

Click here for the latest index numbers in this volatile session. At its lows, the Dow had fallen more than 500 points or 2.1 percent.

The Dow dropped 1,032 points Thursday, its second drop of that magnitude this week. The S&P 500, meanwhile, broke below its 200-day moving average, a key technical level according to traders.

"We bounced off the 200 moving day average on the S&P 500 and saw some algorithmic programmed buying," said FBN Securities's Jeremy Klein, who called for a market downturn in January. "For the most part, the rates going higher triggered the decline … once we get through this bout of volatility, the rates will matter again much more."

The recent turmoil in equities began last Friday, when the Dow fell 666 points after a better-than-expected jobs report ignited inflation fears. That fall was exacerbated Monday after the yield on the benchmark 10-year Treasury note hit a 4-year high, sending the Dow tumbling another 1,175 points as investors grew more nervous about an overheating economy.

Trouble with securities called exchange-traded notes that decline in value when volatility increases likely helped create more turmoil in the markets this week. The Cboe Volatility index (VIX) — the market's best fear gauge — was higher around the 30 level after jumping as high as 50 earlier in the week. At the end of January, the VIX was below 14. Read more at CNBC