The Nifty50 on the National Stock Exchange (NSE) also plummeted around 300 points to trade well below the 8,500 mark.
The decline was in line with the Asian stock markets, which fell due to the growing number of novel coronavirus cases worldwide.
At 11.15 a.m., Sensex was trading at 28,349.09, lower by 1,119.40 points or 3.80 per cent from the previous close of 29,468.49.
It had opened at 29,505.33 and has so far touched an intra-day high of 29,505.98 and a low of 28,341.14.
The Nifty50 was trading at 8,280.95, lower by 316.80 or 3.68 per cent from its previous close.
Heavy selling pressure on the banking and financial stocks pulled the indices down.
Among the Sensex stocks, Kotak Mahindra Bank, Reliance Industries and IndusInd Bank lost the most, while Hero MotoCorp and Bajaj Auto were the only gainers.