A bruising stock selloff continued Monday, erasing more than 650 points from the Dow Jones Industrial Average, as Treasury Secretary Steven Mnuchin tried to instill calm into a jittery market.

Coming off the stock market’s worst week since the 2008 financial crisis, the Dow Jones Industrial Average, S&P 500 and Nasdaq Composite fell for a fourth straight session as investors continued to weigh the impact of rising interest rates, slowing U.S. growth and the ramifications of a government shutdown extending into January.

Monday’s 2.9% drop during the shortened trading day was the Dow industrials’ worst performance ahead of the Christmas holiday, trumping a 1918 record.

With the selloff showing little signs of slowing, Mr. Mnuchin attempted to reassure investors, saying he had spoken individually with the chief executives of six large banks to ensure they had sufficient lending capacity.

But Mr. Mnuchin’s public efforts to soothe investors may have had the opposite effect, analysts said, with bank stocks falling along with most other assets Monday.