In the initial trade, the key indices opened on a flat note and started to fall from the opening.
Globally, Asian stocks fell on Thursday amid concerns that the recovery from the pandemic will slow as elevated inflation forces tighter monetary policy.
On the domestic front, volumes on the NSE were lower than recent average while the advance decline ratio was sharply negative.
Among sectors, only the Capital Goods index traded in the green whereas Realty, Power, Oil & Gas, Metals, Banks, Telecom and FMCG fell the most.
At 12.40 p.m., the 30-scrip sensitive index traded at 60,510.16 points, down 633.17 points or 1.04 per cent.
The Sensex opened at 61,081 points from its previous close of 61,143.33 points.
Besides, the NSE Nifty50 traded at 18,014.75 points, lower by 196.20 points or 1.08 per cent.
It opened at 18,187.65 points from its previous close of 18,210.95 points.
"Morgan Stanley has downgraded Indian markets to equal weight. All these could lead to FPI unwinding and continued pressure on indices," said Deepak Jasani, Head of Retail Research, HDFC Securities.
According to Likhita Chepa Senior Research Analyst at CapitalVia Global Research:
"Indian equity benchmarks made a negative start on Thursday tracking weakness in global peers. Markets quickly extended their losses and are now trading lower, with each market losing more than half a percent in early trade."
"Traders were wary since the October F&O series contract was set to expire later in the day."
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