NEW YORK (MarketWatch) — U.S. stock-index futures slid sharply, Treasury prices rallied and gold futures topped $1,700 an ounce Monday as Wall Street readied to react to the unprecedented downgrade of U.S. debt by Standard & Poor’s.

The rating agency’s decision “is going to test the notion that ‘anticipated’ is always synonymous with ‘fully discounted’ in the world markets,” noted Nicholas Colas, chief market strategist at ConvergEx Group.

Futures on the Dow Jones Industrial Average DJ1U -0.50%  slumped 241 points to 11,161. Futures on the Standard & Poor’s 500 index SP1U -0.46%  dropped 28 points to 1,169.8 and those on the Nasdaq 100 ND1U -0.56%  sank 50 points to 2,137.

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S&P downgrade: What’s the fallout?

Standard & Poor’s downgraded the U.S. government's AAA sovereign credit rating, an unprecedented action that looks like to send shock waves through the global financial system.

Late Friday, Standard & Poor’s downgraded the U.S. government’s AAA debt rating to AA+, and assigned it a negative outlook. The agency said the U.S. political system had become less stable and the deficit-reduction deal reached by lawmakers last week was not extensive enough. Read U.S. triple-A debt rating cut

“The downgrade was expected,” said Peter Cardillo, chief market economist at Rockwell Global Capital. “As the first-ever [U.S. debt downgrade] announcement, it was taken as a shock, but I think it was pretty much priced into the marketplace.”

“Whether or not investors continue to choose to lower risk, I think that’s yet to be seen and I think the real basis for this huge decline has a lot to do with hedge-fund selling, which feeds on itself,” said Cardillo.

The Dow Jones Industrial Average DJIA -5.55%  fell 5.8% last week, its worst weekly performance since March 6, 2009, while the S&P 500 index SPX -6.66%  fell 7.2% on the week, its worst since late in 2008. The Nasdaq COMP -6.90%  finished the week down 8%, its worst since late 2008 as well.

S&P DOWNGRADES U.S. | MarketWatch Topics: Credit

Markets tank after S&P downgrade
Dow ends 635 points lower after S&P downgrade of U.S. credit rating.
• S&P strips coveted U.S. rating
• How downgrade affects investors (SmartMoney)
• China rips U.S. on debt-rating downgrade
• Geithner keeps heat on S&P
• U.S. unlikely to regain AAA rating soon: S&P
• Obama says U.S. will always be Triple-A

NEWS ANALYSIS
• Three trades to make after downgrade
• Reeves: S&P’s downgrade means nothing
• Harrison: What matters and why (Minyanville)
• How to brace a portfolio for another recession
• ‘Optimism index’ suggests recession is here

MULTIMEDIA
• Obama: Need a balanced plan for deficit
• International reactions to U.S.'s cut rating
• GOP hopefuls point fingers for downgrade

News of the rating downgrade provoked sharp losses in Asia and the Middle East. Stocks in Europe also tumbled, with the Stoxx 600 XX:SXXP -4.14%  down 2% in early afternoon trading.

European stocks also slid, giving up gains that came after the European Central Bank said late Sunday it would “actively implement” its bond-buying program in an effort to calm market fears that the euro-area debt crisis will spread to Italy and Spain.

Financial markets seemed to pay little attention to a statement from the Group of 20 nations, which said Monday they stood ready to take “all necessary initiatives” to “ensure financial stability and liquidity in financial markets.” That follows a similar Group of Seven statement on Sunday. Read G-7 seeks to calm market's debt jitters

But Cardillo said there’s a good chance that U.S. stocks could reverse predicted losses later in the day “when the market begins to stabilize.”

Steen Jakobsen, chief economist at Saxo Bank, agreed. “People will start very, very defensively, but if the market keeps momentum into the close, you could see a rally in markets in New York,” said Jakobsen.

“But remember this is clearly a bear-market rally,” he added. “The policy response over the weekend was predictable and expected. What now remains is for policy makers to create more credibility by supporting the words with actions.”

Investors, clearly rattled by the S&P move, rushed to perceived safe-haven investments, such as gold. Gold futures for December delivery GC1Z +0.58%  rallied nearly $51.70 to $1,703.50 an ounce.

Crude-oil futures for September delivery CL1U -0.48%  fell $4.11 to $82.77 a barrel. Oil and energy-related stocks are often perceived as riskier investments.

Dollar Thrifty Automotive Group Inc. DTG -1.33%  said second-quarter net profit rose 0.6% to $42.5 million from $42.3 million as revenue fell 0.3%. Dollar Thrifty also said it doesn’t yet have an agreement on any merger terms with either Avis Budget Group Inc. CAR -11.77%  or Hertz Global Holdings Inc. HTZ +0.25% , though it continues to cooperate with both.

Shares of Berkshire Hathaway Inc. BRK.A -0.59%   BRK.B +0.15%  could be active. Late Friday, the company said second-quarter profit rose to $3.42 billion, or $2,072 per Class A share, from $1.97 billion, or $1,195 per Class A share, in the same quarter last year.

In merger news, a Berkshire Hathaway subsidiary offered to acquire all the outstanding common shares of Transatlantic Holdings Inc. TRH -0.79%  at a price of $52 each. Transatlantic’s board on Sunday advised its shareholders to take no action on the bid, and said it continues to recommend a stock-swap merger agreement struck in June between Allied World Assurance Co. Holdings AWH -1.39%  and Transatlantic.  

Kate Gibson is a reporter for MarketWatch, based in New York. Barbara Kollmeyer is an editor for MarketWatch in Madrid.