Greece is moving closer to defaulting on its debt and could be forced to abandon the euro currency.
The Dow Jones industrial average dropped 350 points, or 2 percent, to 17,596 Monday.
The Standard & Poor's 500 index sank 43 points, or 2.1 percent, to 2,057.
The Nasdaq composite tumbled 122 points, or 2.4 percent, to 4,958.
European markets fell even more. Stocks fell 3.6 percent in Germany and 3.7 percent in France.
Bond prices rose sharply as investors sought safety. The yield on the 10-year Treasury note fell to 2.32 percent.