(Bloomberg) -- U.S. stocks rallied the most since 2011, halting a six-day rout, as investors found some relief after the worst global equity rout in almost four years. Treasuries fell and the dollar rose.

The S&P 500 jumped 3.9% amid a second day of dramatic swings in equity markets, succeeding where a rally yesterday failed in the final hours of trading. Two things that have supported U.S. stocks in the past, dovish words from the Federal Reserve and improving economic data, halted a plunge that erased $2.2 trillion from share values. 

"It's definitely a positive to see markets move higher," said Tom Manning, chief investment officer from Boston Private Wealth, which oversees about $9 billion in assets. "I don't know that we found the bottom. I'm not convinced we don't have more negative days to follow. We're not likely to go from extreme volatility to extreme calm overnight."

Volatility could be seen throughout financial markets. Commodities resumed their decline following a reprieve Tuesday. Gold fell for a third day, the longest stretch in a month, while copper led industrial metals lower and crude traded below $40 a barrel.

CONCERNS ABOUT CHINA

Earlier attempts to push equities higher fell short across the globe. The Shanghai Composite Index ended 1.3% lower despite an early 4.3% surge. European stocks were also whipsawed, erasing a 2.7% decline only to slump again. Canadian equities erased a 1.6% surge in their first half hour of trading.

"We'll have more volatility until we get more visibility," said Jacques Porta, who helps oversee the equivalent of $570 million as a fund manager at Ofi Gestion Privee in Paris. "The Chinese devaluation worried investors and the economic data has struggled. The potential rate increase in the U.S. also gives worry that worldwide economic growth will slow."

Concern that Chinese policy makers may fail to prevent a hard landing in the world's second-largest economy has convulsed global markets. About $8 trillion has been erased from the value of global equities since China's surprise devaluation of the yuan on Aug. 11 as investors weighed prospects for slowing growth and the first interest-rate increase in the U.S. in almost a decade.

The S&P 500 climbed 3.9% at 4 p.m. in New York, the biggest advance since November 2011. A rally in the first few minutes of trading eroded by more than half throughout the morning, before an afternoon rebound took over. Investors yesterday saw a 2.9% rally evaporate in the last hour of trading, sending the gauge plunging for a loss of 1.4%.

The selloff in equities broke a calm in a stock market that had gone almost four years without a 10% correction. The S&P 500 plunged 11% in the six days through Tuesday, the most since the U.S. was stripped of its AAA credit rating by S&P in August 2011, and was 1% away from erasing its gains since the end of 2013.

Technology companies led the gains Wednesday, with Apple Inc., Google Inc. and Intel Inc. rising at least 5%. Cameron International Corp. soared 41% after agreeing to be bought by Schlumberger Ltd. in a $14.8 billion deal.

The Chicago Board Options Exchange Volatility Index slipped 17% to 30.04. The measure of market turbulence known as the VIX declined for a second day after a record six-day jump sent the gauge to its highest level since October 2011.

RATE HIKE POSTPONED?

Global stock-market turmoil has weakened the case for raising interest rates in September, Federal Reserve Bank of New York President William C. Dudley said, cautioning it's important not to overreact to short-term developments.

"From my perspective, at this moment, the decision to begin the normalization process at the September FOMC meeting seems less compelling to me than it was a few weeks ago," Dudley told a news conference Wednesday at the New York Fed.

Treasuries fell, pushing 10-year yields to a one-week high, as a report showed unexpected strength in orders for durable goods while advancing stocks dimmed interest in government bonds. The benchmark U.S. 10-year note yield rose 11 basis points to 2.19%.

The Bloomberg Dollar Spot Index jumped 0.7%, advancing for a second day. The currency gained 1.7% to $1.1328 per euro, and 0.8% versus the yen.

The Stoxx Europe 600 Index lost 1.8% after declining as much as 2.7%, at one point erasing all those losses before tumbling again.

The Shanghai Composite swung during the day, sliding as much as 3.9%, then rallying 4.3%, before closing down 1.3%. Hong Kong's Hang Seng China Enterprises Index also erased earlier gains, falling 0.9%.

Chinese equities have lost half their value since mid-June, as margin traders closed out bullish bets. The government has halted intervention in the equity market this week as policy makers debate the merits of an unprecedented rescue, according to people familiar with the situation.

The MSCI Emerging Markets Index was little changed, after rallying 2.2% on Tuesday, the most in two years. Equity gauges in India, and Saudi Arabia and South Africa lost more than 1%. South Korea's Kospi jumped 2.6%, capping the biggest two-day rally since June 2013, as tensions between North and South Korea eased.

The Bloomberg Commodity Index retreated 1.3%. Copper fell 2.6% and gold lost 1.2% to $1,124.60 an ounce.

West Texas Intermediate fell 1.8% to $38.60 a barrel, having earlier risen as much as 1.4%. Rising U.S. fuel stockpiles and further declines in China's stock market fanned concern that demand may slow while global crude markets remain oversupplied.

--With assistance from Yuko Takeo in Tokyo, Emma O'Brien in Wellington, Neil Denslow, Cecile Vannucci, James Herron, David Goodman, Inyoung Hwang and Stephen Kirkland in London and Kyoungwha Kim and Nick Gentle in Hong Kong.

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