Most likely, sp will go down at the market opening and up in the afternoon.

NEW YORK (CNNMoney) -- U.S. stocks were set to tumble at the open Monday, tracking deep declines in global markets, as investors get their first opportunity to react to Standard & Poor's downgrade of U.S. debt.

S&P 500 (SPX) and Nasdaq (COMP) futures were all down more than 2% ahead of the opening bell. Dow Jones industrial average (INDU) futures were less than 2% lower. Stock futures indicate the possible direction of the markets when they open at 9:30 a.m. ET.

Late Friday, S&P downgraded United States' credit rating by one notch to AA+, removing it from the Triple A-club for the first time in history.

Economists and investment strategists largely agree that there could be an initial shock, but the downgrade itself should not impact markets too much.

S&P rating: How to get back to AAA

Still, the rating cut is unprecedented, so nobody can be certain.

Len Blum, managing partner for Westwood Capital, said the S&P downgrade of U.S. debt is the prime reason why futures are down, because traders are anticipating a sell-off after the opening bell.

However, he said that, "In reality, it should not have affected the markets," because there's no new information in the downgrade.

"The only time a rating agency should move the markets is when they discover something that nobody else has seen," he said. "All they're doing is telling us what we already know, and they're spanking the government."

S&P's move came at the end of a tumultuous week on Wall Street, with all three indexes delivering their worst performances since the darkest months of the 2008-2009 financial crisis.

Stocks plunged Thursday, with the Dow tumbling 512 points, as fear about the global economy spooked investors. Markets ended Friday on a mixed note, when the Dow had a massive trading range of 400 points.

Peter Cardillo, chief market economist for Rockwell Global Capital, said that last week's declines on the stock market might keep Monday's drop in check.

"We're certainly oversold, which means that we should be able to stabilize very shortly," he said.

European debt crisis: Investors will be weighing fresh news out of Europe Monday.

The European Central Bank signaled in a statement Sunday that it was ready to begin buying Italian and Spanish government bonds -- stepping up its efforts to slow the rising panic over the eurozone's debt crisis.

In a separate announcement, finance ministers from the G-7 -- a group of significant world economies -- pledged support for troubled countries.