Global markets continue to react to debt fears, including the first-ever downgrade of the U.S. credit rating by the Standard & Poors rating agency and moves by the European Central Bank to bolster Italy and Spain.

Major indexes across Asia plunged between 2 percent and nearly 4 percent Monday, while stock futures in New York were lower ahead of the market open on Wall Street. The S&P 500 appears set to drop 2 percent at the opening bell.

U.S. leaders have criticized the S&P downgrade, which signals that the agency believes U.S. government bonds are now a riskier investment.

Meanwhile, markets in Italy and Spain rose sharply Monday after the European Central Bank said it would buy government bonds from both countries in an effort to calm fears about the eurozone debt crisis. Major European markets were down slightly after opening.

U.S. Treasury Secretary Timothy Geithner says the S&P used “terrible judgment” when it downgraded the U.S. credit rating last week from the top triple-A grade. Geithner says the S&P showed a “stunning lack of knowledge” about the mathematics used to draw up a federal budget.

S&P Managing Director John Chambers predicts another possible downgrade as soon as six months from now.

But Geithner said the United States has a very resilient and strong economy. He said U.S. treasuries are an absolute safe investment and that there is no risk of the United States not being able to meet its obligations.

The other two major credit rating agencies – Moody’s and Fitch – have so far kept the U.S. triple-A rating.

S&P defended its decision to drop the credit rating. It blamed Congress for months of political haggling over a deficit reduction deal that S&P says does not go far enough. The deal calls for cutting the deficit by more than $2 trillion over 10 years. S&P called for $4 trillion in savings.

Geithner said Sunday he will not resign. He was considering leaving the job when the debt ceiling debate in Congress ended. A White House spokesman said President Obama is pleased that Geithner will remain at his post.

Republican Senator Rand Paul and Congresswoman Michelle Bachmann – a Republican presidential candidate – have both publicly demanded that Geithner quit, holding him and the Obama administration responsible for the lowered credit rating.