The broader Nifty too was stuck below the 9,900 level to trade at 9,855.90.
Geopolitical tensions on North Korea and continuous FII outflows led to nervous moments ahead of derivatives expiry due on Thursday.
The Sensex traded at 31,597.27 at 1238 hours, down 325.17 points, or 1.02 per cent.
The broader Nifty fell 108.50 points, or 1.09 per cent, to 9,855.90 at 1238 hours.
Foreign portfolio investors (FPIs) sold shares worth Rs 1,241.73 crore last Friday, showed provisional data from the stock exchanges. Domestic institutional investors (DIIs) bought shares worth a net Rs 521.17 crore.
Overseas, Asian stocks dropped as investors digested the political developments in Germany and New Zealand over the weekend. Germany's Merkel's party looks set to remain the largest entity in the next German parliament.
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