NEW YORK — Stock prices hurtled lower Monday as anxiety overtook investors on the first trading day since Standard & Poor’s downgraded American debt. The Dow Jones industrials were briefly down more than 600 points.

The Dow fell below 11,000 for the first time since November. The sharp drop extended Wall Street’s almost uninterrupted decline since late July, when the Dow was flirting with 13,000.

Investors worried about the slowing U.S. economy, escalating debt problems threatening Europe and the prospect that fear in the markets would reinforce itself, as it did during the financial crisis in the fall of 2008.

They desperately looked for safe places to put their money and settled on U.S. government debt — even though those were the targets of the downgrade Friday, when S&P removed the United States from its list of the lowest-risk countries.

The price of Treasurys rose, and yields, which move in the opposite direction from price, fell. The yield on the 10-year Treasury note fell to 2.33 percent from 2.57 percent Friday.

“This is largely a flight to safety,” said Thomas Simons, money market economist with Jefferies & Co. “The bond market is really trading off of what’s going on in the stock market.”

Gold set a new record, trading for more than $1,700 an ounce.

The stock market plunged at the opening bell, with the Dow down 250 points in minutes. Stocks steadily fell for most of the rest of the morning and early afternoon, and the Dow was briefly down 600 at about 2:30 p.m.

In afternoon trading, the Dow was down 400 points, or 3.4 percent to 11,079. The S&P 500 was down 51 points, or 4.2 percent, to 1,149. The Nasdaq was down 114 points, or 4.5 percent, to 2,417.

Stock markets in Asia began Monday’s global rout. The main stock index fell almost 4 percent in South Korea and more than 2 percent in Japan. European markets opened later and fell, too, with Germany down 5 percent and France 4.7 percent.

In the U.S., stocks fell even though Moody’s, another major credit rating angecy, stood by its top rating of Aaa for the United States. It said it could downgrade the U.S. if it cut its deficit, “but it is early to conclude that such measures will not be forthcoming.”

Financial markets also did not appear comforted by an afternoon statement by President Barack Obama, who said Washington needs more “common sense and compromise” to tame its debt.

“Markets will rise and fall,” he said. “But this is the United States of America. No matter what some agency may say, we’ve always been and always will be a triple-A country.”

S&P, in its downgrade, criticized dysfunction in the American political system. The downgrade wasn’t a total surprise but came when investors were already feeling nervous about the U.S. economy and European debt, among other problems.

Last week, the Dow Jones industrial average fell almost 700 points. That was its biggest point loss since October 2008, during the financial crisis. Counting Monday, the Dow has dropped in 10 of the last 12 trading days.

Crude oil, natural gas and other commodities fell on worries that a weaker global economy will mean less demand. Oil fell $3.47 to $83.41 per barrel.

S&P on Monday downgraded mortgage lenders Fannie Mae, Freddie Mac and other agencies linked to long-term U.S. debt. Fannie and Freddie own or guarantee about half of all U.S. mortgages. Their downgrade could mean higher mortgage rates.

Worries about weaker profits that could result from a slowing economy have slammed the financial industry since late July. As a group, financial stocks in the S&P 500 index fell 4.9 percent on Monday to their lowest level since July 2009.

Bank of America fell 13.7 percent after AIG filed suit against the bank. The insurer alleged Bank of America sold it overvalued mortgage-backed securities. The bank denied the allegations. Its stock has dropped by nearly 50 percent this year.

Stocks in other industries whose profits are closely tied to the strength of the economy also fell sharply. Energy stocks in the S&P 500 fell 4 percent, for example.

The smallest losses came in safer industries such as consumer staples whose profits tend to be steadier, regardless of the economy. Even in a bad economy people will still buy things like toothpaste and bread.

The Vix index, a measure of fear among investors, shot up 26 percent to its highest level since May 2010. The index shows how worried investors are that the S&P 500 will drop over the next 30 days. It does this by measuring prices for stock options that investors can buy to help protect their portfolios.

Investors are worried that Spain or Italy could become the next European country to be unable to pay its debt. The European Central Bank said it will buy Italian and Spanish bonds in hopes of helping the countries avert a possible default.

Seeking to avert panic spreading across financial markets, the finance ministers and central bankers of the Group of 20 industrial and developing nations issued a joint statement Monday saying they were committed to taking all necessary measures to support financial stability and growth.

“We will remain in close contact throughout the coming weeks and cooperate as appropriate, ready to take action to ensure financial stability and liquidity in financial markets,” they said.

Worries about the U.S. economic recovery have been building since the government said that economic growth was far weaker in the first half of 2011 than economists expected.

The economy grew at a 1.3 percent annual rate from April through June, below economists’ expectations. It expanded at just a 0.4 percent rate in the first quarter. The first half of 2011 was the slowest since the end of the recession.

Then reports showed that the manufacturing and services industries barely grew in July. Job growth was better than economists expected last month. But the 117,000 jobs created in July were still well below the 215,000 that employers added between February and April, on average.

The Federal Reserve will meet on Tuesday, but economists don’t expect much to come out of the meeting. The central bank’s key interest rate is already at a record of nearly zero, where it has been since 2008.

The Fed has also already said that it plans to keep rates low for “an extended period.” Chairman Ben Bernanke said last month that the Fed could step in to help the economy if it further weakened.

Fears about a weaker U.S. economy have overshadowed profit growth that companies have reported for the second quarter. For the 441 companies in the S&P 500 that have already reported, earnings rose 12 percent in the second quarter from a year earlier. Revenue growth has also topped 10 percent for the first time in a year.

Tyson Foods rose 0.8 percent after it reported stronger profit than analysts expected. The largest U.S. meat company said its net income fell 21 percent because of higher grain costs, but analysts expected a steeper drop.

Tyson was one of just five stocks in the S&P 500 to rise on Monday. The biggest gain came from Newmont Mining Corp., which benefited from higher prices for the gold that it produces.

Verizon Communications Inc. fell 3.9 percent after it was unable to come to terms with 45,000 workers on health care costs, pensions and other issues.