* U.S. stock futures, dollar, oil fall on U.S. downgrade
* Euro rises on hopes ECB will buy Italian, Spanish bonds
* Some global investors question S&P downgrade
* U.S. downgrade exacerbates growth worries (Adds reaction in U.S. stock index futures, other assets)
By Ryan Vlastelica and Gertrude Chavez-Dreyfuss
NEW YORK, Aug 7 (Reuters) - U.S. assets fell sharply in early electronic trading on Sunday in response to a downgrade of the U.S. credit rating by Standard & Poor's, while the euro rose on expectations of further bond purchases by the European Central Bank to deal with the euro zone's debt crisis.
S&P 500 stock futures
"We're set up for a pretty substantial selloff, with further erosion of the market as time goes on," said Jeffrey Sica, president and chief investment officer at SICA Wealth Management in Morristown, New Jersey. "There's a real decline in confidence right now."
Among other assets, crude oil slumped 3.6 percent while gold -- typically viewed as a safe-haven investment -- climbed 2.5 percent. The U.S. dollar index <.DXY> fell 0.3 percent while the euro jumped.
Reaction in the U.S. government bond market was mild, with 30-year long bond futures down moderately at the start of electronic trade in Asia and 10-year futures up slightly.
The 30-year T-bond future
Following a conference call held by the European Central Bank on Sunday, a euro zone monetary source said the ECB will intervene "significantly" to protect Italy and Spain from the debt crisis, indicating it would buy government bonds of the euro zone's third and fourth biggest economies.
A statement from the ECB said it would "actively implement" its bond-buying programs. [ID:nL6E7J704K]
The U.S. dollar took a further beating against the safe-haven Swiss franc and Japanese yen, though markets were aware of the possibility of intervention by the Bank of Japan and Swiss National Bank to stem their surging currencies.
The dollar fell against the Swiss franc to 0.7570 franc from 0.7671 on Friday. It also lost ground against the yen to 78.01 yen from 78.38 on Friday.
The move by S&P drew criticism from some of the world's largest investors. For the S&P story, click on [ID:nLDE77500Z]
"Obviously, we're going to get freaked out a little bit and the dollar will get hit, but it's only going to be for a couple of days," said John Taylor, chairman and chief executive officer of FX Concepts, the world's largest currency hedge fund.
"This downgrade is not that important and if you ask me, too silly. The U.S. is in a much better position than any, I repeat, any European country," Taylor added.
While the downgrade was seen as compounding uncertainty in Europe, it was not yet clear whether European policymakers would be able to come up with measures to allay concerns about their own region's crisis. However, all the signs were that they were keenly aware of the importance of reassuring markets.
Germany and France on Sunday reiterated their commitment to implementing the decisions of last month's emergency EU summit. [ID:nLDE77607W] [ID:nL6E7J704K]
One ECB source said that if the ECB council opted to intervene on Italy, the ECB and national central banks would start buying Italian bonds when markets open on Monday.
The ECB last week resumed its purchases of government bonds in the secondary market after an 18-week hiatus, but its decision to restrict such purchases to Irish and Portuguese bonds led to sharp declines in Italian and Spanish bond prices, and borrowing costs soared to 14-year highs.
Any ECB buying would offer relief to beaten-down Italian and Spanish bonds, although the extent of any rally in these bonds will depend on the size and persistence of the bank's bond purchases.
U.S. RECESSION FEARS
Worries of another U.S. recession and concern about the euro zone crisis have sparked a global stock market slump that wiped $2.5 trillion off companies' values in the past week.
The fall in global share prices, as measured by the MSCI All-Country World Index <.MIWD00000PUS>, was the biggest weekly decline since early October 2008, according to Thomson Reuters Datastream.
Some expressed doubts about the S&P decision as they are well aware of questions on the S&P's calculations of the projected U.S. fiscal deficits. [ID:nN1E774236]
"The U.S. track record -- over the past 200 years -- on its ability and willingness to fully service its debt is impeccable and the debt statistics should be interpreted not in isolation but in conjunction with the flawless track record of the U.S.," said Stephen Jen, managing director of SLJ Macro Partners in London, a global macro hedge fund.
"This will have no lasting effects on financial asset prices," he added. <^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Graphic on week's losses http://link.reuters.com/xus92s Euro zone crisis graphics http://r.reuters.com/hyb65p Euro zone bond spreads http://r.reuters.com/kus82s Dealers cut U.S. growth outlook [ID:nN1E7741Y9] ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^>
The sharp swings in financial markets have piled pressure on policymakers. Finance ministers from the Group of Seven most developed economies were discussing the U.S. sovereign rating downgrade and Europe's debt woes late on Sunday.
"Be wary (Monday) of irrational depression as markets take flight," said Justin Urquhart Stewart, a director at Seven Investment Management in London. "We are dealing with the knowns and not the unknowns, but what we have a shortage of at the moment is political leadership."