* Citigroup tumbles more than 23 percent.

* December U.S. retail sales fall 2.7 percent.

* Dow <.DJI> falls 248.34 points, or 2.94 percent, to 8,200.22; the S&P 500 <.SPX> slides 29.12 points, or 3.34 percent, to 842.67; the Nasdaq Composite <.IXIC> sheds 56.82 points, or 3.67 percent, to 1,489.64.

(Editing by Leslie Adler)