NEW YORK -- U.S. investors will have their first chance today to react to Standard & Poor's decision to strip the U.S. government of its top credit rating. But the bigger issues facing Wall Street and stock markets worldwide remain debt-ridden countries in Europe and concerns that the global economy is weakening.

The downgrade of U.S. long-term debt from AAA to AA+ wasn't unexpected and may have little effect on interest rates. But it's the kind of news that stock markets don't need when investors are nervous. As a result, financial analysts interviewed Sunday said they expect markets to be volatile this week -- and beyond.

That view was echoed by former Federal Reserve chairman Alan Greenspan, who appeared Sunday on NBC's "Meet the Press." "It is very unlikely that isn't going to take a while to bottom out," he said of selling in the markets.

Beyond the downgrade, though, investors have plenty of reason to be selling. Last week, the Dow Jones industrial average fell nearly 700 points, or 6 percent. Investors were worried because the economic signals in the U.S. and overseas were pointing toward trouble:

* On July 29, the government lowered its estimate of how much the economy grew during the first quarter. It had said the economy grew at an annual rate of 1.3 percent, but revised that number down to 0.4 percent. That meant the economy barely grew. Second-quarter growth was also weak, 1.3 percent.

* The first reports on the economy during the third quarter have been mixed. Manufacturing, which helped pull the economy out of the recession, fell to its weakest level since July 2009. That was the month after the recession officially ended. The Labor Department said 117,000 jobs were created last month. But that came after 99,000 jobs were created in May and June combined -- and 250,000 new jobs are needed each month to reduce unemployment.

* European officials are trying to help Italy avoid the kind of bailouts that Greece, Portugal and Spain were forced to accept to prevent them from defaulting on their debt. And those bailouts haven't solved all the problems in those countries.

To investors, the downgrade made it all worse.

"We are in unchartered territory and, therefore, should all brace for volatility over a number of days if not weeks," said Mohamed El-Erian, CEO and co-chief investment officer of the bond mutual fund company PIMCO.

Greenspan noted that S&P had "hit a nerve" with its downgrade. The ratings company said it was lowering the U.S. rating not just because of the country's debt load but because S&P doesn't believe Congress has the ability to resolve the country's debt problems. And it warned that another downgrade could be coming.

On Saturday, David Beers, S&P's global head of sovereign ratings, said his company was concerned about "the degree of uncertainty about the political policy process" in Washington.

S&P was looking for $4 trillion in budget cuts over 10 years. The deal that Congress passed Tuesday would bring $2.1 trillion to $2.4 trillion in cuts over that time. S&P said it was also concerned about the ability of Congress to implement those cuts because of the division between Republicans and Democrats.

"Right now, the markets don't believe anybody anywhere and the uncertainty premium is very high. Since the end of World War I, the United States has been an unquestioned AAA credit, until now," said David Kotok, chairman and chief investment officer of Cumberland Advisors.

Prudential Financial market strategist Quincy Krosby said, "The rating is in essence an indictment of Congress and puts the president on the defensive."

Investors are worried about debt not only because countries and many people are overwhelmed by it. Debt is what financed economic growth for decades. Now countries and people are cutting back on debt, and that means economic growth in the future will be slower.

Investors may get more insight Tuesday. The Federal Reserve holds a regularly scheduled meeting on the economy and interest rates. It's expected the central bank will state that interest rates will need to remain at their current low levels for at least another year.

Even with this bleak outlook, some analysts see a chance for stocks to rise, at least in the short run.

The stock market could recover next week if European leaders make progress in averting another debt crisis in that region, said Ryan Detrick, senior technical strategist at Schaeffer's Investment Research.

Still, even if stocks do rise, there are so many economic and political problems to be resolved that any rally may well be very short-lived.