U.S. stock index futures tumbled Friday as results from Thursday's referendum showed the U.K. had voted to leave the European Union (EU).
Dow Jones futures broke below an implied open down more than 700 points overnight, before paring losses to show an implied opening decline of about 500 points.
The referendum result stunned markets, which had expected the U.K. to vote to remain in the EU. The leave camp secured 51.9 percent of the vote.
At around 3:30 a.m. ET, U.K. Prime Minister David Cameron, who campaigned for the remain vote, announced his intention to resign by October.
The U.S. Securities and Exchange Commission's limit-down rule comes into effect when equity securities fall by 5 percent or more. This is designed to control market volatility by preventing trade in securities after large and sudden price moves.