‍Wall Street posted worst ever Christmas Eve drop on Monday, with the S&P 500 tumbled to the brink of a bear market, and oil prices plunged more than six percent to the lowest level in more than a year.

All three major indexes ended down more than two percent the day before the Christmas holiday. The S&P 500 finished about 19.8 percent below its September 20 closing high, just shy of the 20 percent threshold commonly used to define a bear market.

U.S. Treasury Secretary Steven Mnuchin called top bankers on Sunday amid the pullback in stocks and said he was calling a meeting of financial regulators to discuss ways to ensure "normal market operations."

But investors said his move to convene a call with the President's Working Group on Financial Markets, known as the "Plunge Protection team," may have weighed on sentiment on Monday.

"When the Dow is down 600 points, it's hard to say it was a positive," said J.J. Kinahan, chief market strategist at TD Ameritrade in Chicago. "Although his intention was a very good one, the net feeling I think was, Is there a bigger problem that we don't know about?"

Investors also were grappling with the federal government shutdown and reports that President Donald Trump privately discussed the possibility of firing the Federal Reserve chairman.

"The only problem our economy has is the Fed," Trump lashed out at the Federal Reserve Monday morning. "They don't have a feel for the Market, they don't understand necessary Trade Wars or Strong Dollars or even Democrat Shutdowns over Borders."

"The headlines we are seeing today, yesterday, over the weekend are not great," said Vinay Pande, global head of trading strategies at UBS Global Wealth Management.

"The market is concerned about what is happening in D.C.," he said, adding that "In the face of a large correction in the market, there seems to be disarray and disunity and people aren't speaking with one voice, which I think is discouraging to anybody in the market."

The Dow Jones Industrial Average fell 653.17 points, or 2.91 percent, to 21,792.2, the S&P 500 SPX lost 65.52 points, or 2.71 percent, to 2,351.1 and the Nasdaq Composite dropped 140.08 points, or 2.21 percent, to 6,192.92.

Last week, the S&P 500 suffered its biggest weekly percentage drop since August 2011, while the Dow had its biggest weekly drop since October 2008.

All 11 S&P 500 sectors finished down on the day on Monday, and all were in negative territory for the year. The index posted its lowest closing level since April 2017.

Roughly three-fourths of the S&P 500 was trading in the bear market territory. All 30 components of the Dow industrials finished in the red on Monday.

For the third straight day, more than 2,600 New York Stock Exchange- and Nasdaq-listed stocks hit 52-week lows, reflecting a depth of selling not seen in the market since the height of the financial crisis a decade ago.

U.S. crude futures CLc1 and global benchmark Brent LCOc1 hit their lowest levels since 2017 during the session, putting both benchmarks on track for losses of about 40 percent in the fourth quarter.

"What's happening in the stock market is raising fears that the economy is grinding to a halt and thereby will basically kill any future oil demand," said Phil Flynn, an analyst at Price Futures Group in Chicago. "They're pricing in a slowdown in the economy if not a recession with this drop."

The fourth-quarter price decline is likely to cause producers to throttle back on their output, said Flynn.

(With inputs from Reuters)