Harvard University economist Lawrence Summers warned that the stock market turbulence is likely to continue, likening last week’s $5.4 trillion market wipeout to 1987’s Black Monday.

“This was the fourth-largest two-day move since the second World War,” Summers wrote on X.

“The other three were the 1987 crash, the 2008 financial crisis, and the COVID pandemic,” said the former Treasury Secretary under the Clinton administration.

On "Black Monday," October 19, 1987, the Dow Jones Industrial Average plunged 508 points, or 22.6%, the largest one-day percentage drop in its history.

“A drop of this magnitude signals that there’s likely to be trouble ahead, and people ought to just be very cautious,” continued the former National Economic Council director for President Barak Obama.

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U.S. and global stock markets sank last week after President Donald Trump imposed hefty retaliatory tariffs on China, the European Union and a slew of other nations, on top of a 10% levy on all U.S. imports. The S&P 500 Index declined to its lowest level in 11 months, slashing $5.4 trillion in market value in just two trading sessions.

In the three trading days following Trump's announcement of broad reciprocal tariffs on almost all countries, stock markets across the world have nosedived, while investors have barreled into U.S. government bonds as both a safe haven and a bet on Federal Reserve rate cuts.

Some hedge funds are offloading all or most of their stock holdings as the Trump trade war heats up.

In premarket trading Monday, U.S. stock index futures continued to plummet, and the S&P 500 looked set to confirm a bear market. Nasdaq entered a bear market on Friday.