The Dow Jones industrial average plunged 216 points to 13,784 Monday, a loss of 1.6 percent and the biggest drop since November.
The Standard & Poor's 500 dropped 27 points, or 1.8 percent, to 1,487. The Nasdaq lost 45 points to 3,116.
An early gain was gone by midday after reports from Italy suggested that the country was headed for political gridlock following strong gains by former premier Silvio Berlusconi and a protest campaign led by a former comedian.
Four stocks fell for every one that rose on the New York Stock Exchange. Volume was average, 3.8 billion shares.