The markets saw profit-booking around the psychological levels of 20k.
The Sensex shed nearly 300 points from the day's high to end below the 20k mark at 19956, weaker by 148 points, and the Nifty ended at 5991, down 39 points.
The midcap index ended at 8103, lower by 54 points and the smallcap index ended at 10234, down 49 points.
A soft Europe weighed on the sentiment back home.
Most European stocks surrendered early gains, with shares of the Swedish fashion retailer Hennes & Mauritz falling sharply, after the company's third-quarter earnings disappointed investors.
The key benchmark indices in the UK, France and Germany were down upto half a percent each in mid-day trades. And the nervousness was further exaceberated as the September derivatives
There was encouraging news elsewhere though. Wall Street edged higher overnight as corporate deal news and earnings expectations continued to fuel investor optimism.
The Dow gained 32 points, or 0.30% to 10,844. And Asian markets hit two-year highs after poor US data reinforced expectations the US Federal Reserve will take more action to help the struggling US economy.
The key benchmark indices in Hong Kong, Indonesia, Singapore, Taiwan and South Korea between 0.33% and 1.22%.
