NEW YORK—Investors fled risk assets far and wide in Monday's stock-market rout.

"Everybody is looking for whatever they perceive as a safe haven, even if it's just plain illogical," said David Kelly, chief market strategist for J.P. Morgan Funds. "Things are pretty dismal right now."

The Dow Jones Industrial Average sank 634.76 points, or 5.55%, to 10809.85, falling beneath 11000 for the first time since November and adding to last week's steep losses.

The blue-chip measure ended exactly on session lows in the biggest single-day point loss since Dec. 1, 2008. It was the index's sixth-biggest point drop in its history, reflecting a toxic brew of investor fears over government debt and the chances that the economy slides into another recession.

Investors traded 9.71 billion shares in New York Stock Exchange composite volume, the fourth-largest single-day total in history.