The dollar and U.S. government bonds are rising as traders move into assets seen as being safe.
The Greek government shocked financial markets with news that it would put its cost-cutting plan to a popular vote, which could lead the country to default on its debt.
Shortly after the opening bell Tuesday, the Dow Jones industrial average is down 250 points, or 2 percent, to 11,699. The S&P 500 is down 27, or 2.2 percent, to 1,225. The Nasdaq is down 79, or 2.9 percent, to 2,605.
Banks fell hard. Both Citigroup and Morgan Stanley are both down more than 9 percent.