All three main indexes fell at least 1.5 percent in the wake of Thursday's referendum that has roiled global markets and led investors to seek safe-haven assets.

The Nasdaq dropped more than 2 percent, underperforming the other major indexes, amid fears that fallout from Britain's decision could hit business investment spending in the technology sector.

Along with tech, materials .SPLRCM, financials .SPSY and energy .SPSY were the worst-performing sectors.

"The momentum has continued downward because there continues to be a lot of uncertainty," said Eric Kuby, chief investment officer at North Star Investment Management in Chicago. "It's important to note that it's orderly. It doesn't feel panic-inspired."