The Sensex was quoted at 26,311.05, down 1,055 points at 1045 hours as against Friday's close of 27,366.07.
The market expected to remain volatile in the near future as traders roll over their positions in the futures & options (F&O) segment from the near month August series to September. The near month derivatives is set to expire on Thursday.
The 50-share Nifty also dropped by 334.55 points or 4.03 per cent to 7,965.40 at 1045 hrs.
Globally, Asian markets were deep in the red on worries over weakness in Chinese economy after a private survey on Friday showed that nations factory output fell to multi-year lows.
Key benchmark indices in Japan, Singapore, South Korea and Taiwan were down 3.20 per cent to 4.60 per cent. Indices in China and Hong Kong was down by 8.45 per cent and 4.45 per cent, respectively.
