The dollar index rose 0.23 percent, with the euro down 0.12 percent to $1.223.
As on other days this week, the losses followed some initial gains, raising the market's main gauge of volatility, the CBOE Volatility Index to 36.13 points, three times what it was a week ago.
The S&P 500 is off around 50 points at 2,631, while the tech heavy Nasdaq index shed 149 points at 6,902. The Dow and the S&P 500 are each down just over 10%, respectively, from their all-time highs.
Most notably, the Dow Jones Industrial Average, an index of 30 leading United States shares, suffered two days of 1,000-point-plus declines.
Global markets have been on edge all week following a huge plunge in United States stocks Monday that was followed by rollercoaster moves on Tuesday and Wednesday and another big decline Thursday.
The S&P 500 posted no new 52-week highs and 47 new lows; the Nasdaq Composite recorded 17 new highs and 208 new lows.
World share markets fell on Friday, with U.
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The global sell-off began last week after a solid USA jobs report fuelled expectations that the Federal Reserve would need to raise interest rates faster than expected, because of the strength of the economy.
The Dow briefly sank 500 points in afternoon trading after surging more than 349 points earlier in the day.
Martin, of Globalt Investments, said he did not see anything specific moving the market lower Thursday.
The pan-European FTSEurofirst 300 index lost 1.66 percent and MSCI's gauge of stocks across the globe shed almost 1 percent.
The Dow Jones industrial average had been down 500 points before swinging to a 330-point gain.
While 1,000-point drops are eye-catching, they don't mean a great deal.
Asia's biggest market - Japan's Nikkei 225 index - lost 2.3 per cent.
In commodities trading, crude oil futures are slumping $0.86 to $60.29 a barrel after sliding $0.64 to $61.15 a barrel on Thursday.
"Higher rates are going to slow the economy, we just don't know when and we don't know which rates to watch, and I think that's the debate that's now playing out in the market", he said.
It's the second time this week it has fallen over 1,000 points. Friday's trading saw an 849-point trading range for the Dow.