Overnight Dow Jones industrial average futures were down by 183 points as of 2:28 a.m. ET, signaling the October stock market drop that intensified on Wednesday could continue on Thursday. Futures implied the Dow will open Thursday down by 274.74 points.
Overnight S&P 500 futures, for their part, were lower by 0.63 percent and continued to point to a decline at Thursday's open.
The SPDR S&P 500 ETF fell 0.57 percent in after hours trading Wednesday, after falling 3.2 percent during a day which saw stocks sell off throughout the afternoon.
"October weakness intensified today as volume surged and selling was widespread," said Bruce Bittles, chief investment strategist at Baird, in a note after Wednesday's close. "We may need to see evidence of exhaustive selling, increased investor pessimism and a healthier breadth back-drop to suggest that near-term risks are ebbing."
The Dow Jones Industrial Average closed 831.83 points lower at 25,598.74 as Intel and Microsoft fell more than 3.5 percent each. The Nasdaq Composite plummeted 4 percent to 7,422.05 as Amazon and chip stocks declined. It was the worst for the Dow and S&P since February.
At the same time, the most widely watched measure of investor fear spiked on Wednesday. The CBOE Volatility Index, popularly known as the VIX, leaped about 44 percent to 22.96 —its highest level since the beginning of April. The VIX measures implied volatility on S&P 500 index options.
The Invesco QQQ Trust, which tracks the Nasdaq-100, fell 0.7 percent in after hours trading Wednesday, signaling tech stocks would be under pressure once again in Thursday's session.
The dropped 3.3 percent to 2,785.68, with the tech sector underperforming. The broad index also posted a five-day losing streak — its longest since November 2016 — and fell below its 50-day and 100-day moving averages, widely followed technical levels.
"While seasonal tendencies become increasingly favorable moving toward year-end, we have yet to see evidence that overall selling has reached downside exhaustion," Bittles said. "For now, caution remains warranted."
Both the Dow and S&P 500 posted their biggest one-day drops since early February, while the Nasdaq notched its largest single day sell-off since June 24, 2016.
Stocks have fallen sharply this month. For October, the S&P 500 and the Dow are down more than 4.4 percent and 3.3 percent, respectively. The Nasdaq, meanwhile, has lost more than 7.5 percent.