The Sensex resumed slightly higher at 18,920.90 and moved up further to 18,930.86, but declined immediately to 18,760.41 before quoting at 18,792.94 at 1015 hours, showing a net loss of 125.58 points, or 0.66 per cent, from its last weekend's level.
The NSE 50-share Nifty also declined by 40.15 points or 0.70 per cent to 5,679.55 at 1015 hours.
Major losers were Sterlite Ind (2.79 per cent), Jindal Steel (2.54 per cent), Bajaj Auto (2.43 per cent), Hindalco Ind (2.22 per cent), HUL (2.08 per cent), Larsen (2.05 per cent) and ITC (1.91 per cent).
The Chinese government announced fresh measures to take the heat out of the nation's property sector, including higher down-payments and mortgage rates.
Meanwhile, foreign institutional investors (FIIs) bought shares worth a net Rs 626.89 crore on last Friday as per the provisional data from the stock exchanges.