It had last ruled below this level on September 18.
After losing 383 points in the previous session as Reserve Bank of India unexpectedly hiked repo rate, the Sensex lost another 388.46 points, or 1.91 per cent, to 19,875.25 at 1300 hrs, led by financial and interest-sensitive sectors. The gauge traded below 20,000 level on September 18.
Similarly, the National Stock Exchange index Nifty dropped by 128.25 pints, or 2.13 per cent, to 5,883.85.
They said absence of directions from the overseas bourses, with Japan and Hong Kong markets being closed for a public holiday, also forced funds to reduce their positions.