S&P cut the long-term debt rating for the U.S. by one notch to AA+ from AAA late Friday. The move wasn't unexpected, but it comes when investors are already feeling nervous about a weak U.S. economy, European debt problems and Japan's recovery from its March earthquake.
Ahead of the opening bell, Dow Jones industrial futures fell 200 points, or 1.8 percent, to 11,202. S&P 500 futures fell 24, or 2.percent, to 1,173. Nasdaq 100 futures fell 44, or 2 percent, to 2,143.
Prices for U.S. government debt, though, rose. That's because Treasurys are still seen as one of the world's few safe investments. The yield on the 10-year Treasury note fell to 2.5 percent from 2.57 percent late Friday. It fell as low as 2.46 percent earlier Monday. A bond's yield drops when its price rises.
"We will learn more about the future path of Treasury prices at today's close than we will by the open," he said. "I want to see how the market clears and how it synthesizes the cacophony of news of late."
Gold is another investment that investors traditionally run to for safety. It rose above $1,700 per ounce for the first time. Its price remains below its 1980 record after adjusting for inflation.
"Investors are concerned about a rising risk of global recession, credit downgrades especially now in the eurozone, such as France, the threat of a major bank bust and a global liquidity trap as investors stay in cash," said Neil MacKinnon, global macro strategist at VTB Capital.
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.
Crude oil, natural gas and other commodities fell on worries that a weaker global economy will mean less demand. Oil fell $3.40 to $83.48 per barrel.
Last week, the Dow Jones industrial average fell 698.63 points. That was its biggest point loss since October 2008, during the financial crisis. The Dow has dropped in nine of the last 11 trading days.
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 between April and June, below economists' expectations of 1.7 percent. It expanded at just a 0.4 percent rate in the first quarter.
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."
The central bank finished a $600 billion program in June to buy Treasurys in hopes of supporting the economy. Chairman Ben Bernanke said last month that the Fed would step in to help the economy if it further weakened. But some Fed policymakers oppose more bond purchases, saying it could lead to higher inflation.